brcmfmac: change return type for brcmf_rollback_toq() to void

The function brcmf_rollback_toq() is already called in error path
and its result should not override the initial error value. As the
function releases the sk_buff there is no need to return anything
so change return type to void.

Reviewed-by: Hante Meuleman <meuleman@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>