brcmsmac: handle packet drop during transmit correctly

brcmsmac: handle packet drop during transmit correctly

commit c4dea35e34f5f46e1701156153a09cce429d1ea9 upstream.

The .tx() callback function can drop packets when there is no
space in the DMA fifo. Propagate that information to caller
and make sure the freed sk_buff reference is not accessed.

Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>