p54: fix a NULL pointer dereference bug

If the RSSI calibration table was not found or not parsed properly,
priv->rssi_db will be NULL, p54_rssi_find needs to be able to deal
with that.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>