brcmfmac: absorb brcmf_sendpkt into brcmf_netdev_start_xmit

brcmf_sendpkt only called by brcmf_netdev_start_xmit now. Absorb it
to increase readability.

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