wl1251: configure hardware en-/decryption for monitor mode

Disable hardware encryption (DF_ENCRYPTION_DISABLE) and decryption
(DF_SNIFF_MODE_ENABLE) via wl1251_acx_feature_cfg while monitor interface is
present.

Signed-off-by: David Gnedt <david.gnedt@davizone.at>
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>