brcm80211: fmac: simplify the brcmf_sdioh_request_buffer() function

The function is only called with sk_buff parameter being non-zero so
the prototype does not need to support passing a char buffer any longer.
When the function is called with a NULL sk_buff parameter it returns
-EINVAL now.

Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Alwin Beukers <alwin@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>