staging: brcm80211: remove custom string library again

There was a clean up commit for softmac driver. Do the same for fullmac
implementation.

Here:
 - strtoul and bcm_strtoul are changed to simple_strtoul
 - bcmstrtok -> strsep

All unused functions are deleted.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>