staging: brcm80211: replace simple_strtoul usage in brcmfmac

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

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>