ath5k: fix memory leak on buf on failed eeprom read

The AR5K_EEPROM_READ macro returns with -EIO if a read error
occurs causing a memory leak on the allocated buffer buf. Fix
this by explicitly calling ath5k_hw_nvram_read and exiting on
the via the freebuf label that performs the necessary free'ing
of buf when a read error occurs.

Detected by CoverityScan, CID#1248782 ("Resource Leak")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>