ath9k_hw: ignore eeprom magic mismatch on flash based devices

Many AR913x based devices (maybe others too) do not have a valid EEPROM
magic in their calibration data partition.

Fixes: 6fa658fd ("ath9k: Simplify and fix eeprom endianness swapping")
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>