brcmfmac: define pr_fmt in one place

Several source files (but not all) define the pr_fmt() macro in exactly
the same way. Instead this commit defines it in a header file so driver
logging is consistent.

Cc: Joe Perches <joe@perches.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>