ath5k: initialize default noise floor

Initialize noise floor variable with a default of -95. This was used
uninitialized in the signal strength (RSSI -> dBm) conversion until the first
noise floor calibration was completed.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>