docs: net: ieee802154.rst: fix C expressions

There are some warnings produced with Sphinx 3.x:

	Documentation/networking/ieee802154.rst:29: WARNING: Error in declarator or parameters
	Invalid C declaration: Expecting "(" in parameters. [error at 7]
	  int sd = socket(PF_IEEE802154, SOCK_DGRAM, 0);
	  -------^
	Documentation/networking/ieee802154.rst:134: WARNING: Invalid C declaration: Expected end of definition. [error at 81]
	  void ieee802154_rx_irqsafe(struct ieee802154_hw *hw, struct sk_buff *skb, u8 lqi):
	  ---------------------------------------------------------------------------------^
	Documentation/networking/ieee802154.rst:139: WARNING: Invalid C declaration: Expected end of definition. [error at 95]
	  void ieee802154_xmit_complete(struct ieee802154_hw *hw, struct sk_buff *skb, bool ifs_handling):
	  -----------------------------------------------------------------------------------------------^
	Documentation/networking/ieee802154.rst:158: WARNING: Invalid C declaration: Expected end of definition. [error at 35]
	  int start(struct ieee802154_hw *hw):
	  -----------------------------------^
	Documentation/networking/ieee802154.rst:162: WARNING: Invalid C declaration: Expected end of definition. [error at 35]
	  void stop(struct ieee802154_hw *hw):
	  -----------------------------------^
	Documentation/networking/ieee802154.rst:166: WARNING: Invalid C declaration: Expected end of definition. [error at 61]
	  int xmit_async(struct ieee802154_hw *hw, struct sk_buff *skb):
	  -------------------------------------------------------------^
	Documentation/networking/ieee802154.rst:171: WARNING: Invalid C declaration: Expected end of definition. [error at 43]
	  int ed(struct ieee802154_hw *hw, u8 *level):
	  -------------------------------------------^
	Documentation/networking/ieee802154.rst:176: WARNING: Invalid C declaration: Expected end of definition. [error at 62]
	  int set_channel(struct ieee802154_hw *hw, u8 page, u8 channel):
	  --------------------------------------------------------------^

Caused by some bad c:function: prototypes. Fix them.

Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>