net/mac80211/debugfs: Convert to kstrou8_from_user

This patch replaces the code for getting an number from a
userspace buffer by a simple call to kstrou8_from_user.
This makes it easier to read and less error prone.

Since the old buffer was only 10 bytes long and the value is masked by a
nibble-mask anyway, we don't need to use kstrtoul but rather kstrtou8.

Kernel Version: v3.0-rc2

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>