cfg80211: Fix signal_type comparison

signal_type is enum cfg80211_signal_type.

This fixes the gcc warning:

scan.c: In function `cfg80211_inform_bss':
scan.c:518:6: warning: comparison between `enum cfg80211_signal_type' and `enum nl80211_bss'
scan.c: In function `cfg80211_inform_bss_frame':
scan.c:574:6: warning: comparison between `enum cfg80211_signal_type' and `enum nl80211_bss'

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>