staging: brcm80211: replace simple_strtoul usage in brcmsmac

The usage of simple_strtoul is not preferred. Instead kstrtoul
should be used. This patch fixes this for the brcmsmac driver.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>