ath9k: add EEPROM offset to debug message

Show the EEPROM offset of the failed read operation
in 'ath9k_hw_nvram_read'. The debug message is more
informative this way.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>