ath9k_hw: Fix bug in eeprom data length validation for AR9485

The size of the eeprom data is 1088 bytes for AR9485. But
a sanity check is done against 4K which would result in a
'potential read past the end of the buffer' smatch complaint.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>