brcmfmac: Remove unnecessary check from start_xmit.

The brcmf_netdev_start_xmit checks if the ndev is still valid by
checking if it still exists in database. This check is not needed
and therefor removed.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>