brcmfmac: simplify transmit path

When getting a transmit packet from the networking layer simply
enqueue the packet unconditional and have it handled by the dequeue
worker. The transfer of the packet to the bus-specific driver part
is now done from one context.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>