ath9k: remove unnecessary code

The array field eeprom_data in struct th9k_platform_data
is a fixed size array so it can never be NULL.

Addresses-Coverity-ID: 1364903
Cc: Arend Van Spriel <arend.vanspriel@broadcom.com>
Cc: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>