staging: r8188eu: use list_head for xmitframe list

The r8188eu driver defines a struct __queue that contains a list_head and
a spinlock.

struct tx_servq describes a station for which we have pending tx packets.
This struct contains a __queue for the pending packets (a __queue of
struct xmit_frame entries). In this particular case, the queue's spinlock
is not used, we need only the list_head.

rtw_dequeue_xframe uses a local variable for such an xmit_frame queue.
This patch replaces this variable with a list_head.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20230123205342.229589-17-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>