p54: avoid uninitialized variable warning for freq

  CC [M]  drivers/net/wireless/p54/eeprom.o
drivers/net/wireless/p54/eeprom.c: In function ‘p54_parse_rssical’:
drivers/net/wireless/p54/eeprom.c:494:8: warning: ‘freq’ may be used uninitialized in this function

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