brcmfmac: avoid error output on header only packet

During SDIO layer flow control signalling firmware can issue
invalid packets. Prevent printing of parsing errors in such case.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Arend van Spriel <arend@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>