brcmfmac: do not reconfigure refill rx on 0-length packet.

When USB device gets removed rx complete comes with 0-length
packets. Do not refill those packets. In some rare cases it can
cause infinite loop.

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: John W. Linville <linville@tuxdriver.com>