brcmfmac: Move common BCDC code in single function.

brcmfmac: Move common BCDC code in single function.

The BCDC functions query_dcmd and set_dcmd both create a msgbuf to
be sent to dongle this code is very similar and for optimisation
best put in a function.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@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>