p54: Fix potential concurrent access to private data

Experience with the rtl8187 driver has shown that mac80211 can make
calls to the config callback routine in rapid succession. This patch
creates a mutex that protects the private data in several of the routines
called by mac80211.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>