zd1211rw: fix invalid signal values from device

Driver uses IEEE80211_HW_SIGNAL_UNSPEC and so signal values reported to
mac80211 should be in range 0..100. Sometimes device return out of range
values. These out of range values can then trigger warning in
cfg80211_inform_bss_frame.

This patch adds checks to enforce range returned from driver to
mac80211 be in 0..100 range.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>