brcmfmac: make debug module parameter more clear

The module parameter definition for brcmf_msg_level has been
reworked to a named module parameter with description so
modinfo is a bit more informative:

parm:           debug:level of debug output (int)

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>