staging: rtl8188eu: remove unused definitions from ieee80211.h

MGMT_QUEUE_NUM, ETH_TYPE_LEN and PAYLOAD_TYPE_LEN are defined but
not used in the driver code, so remove them.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>