ath9k_hw: add support for reading EEPROM data from the internal OTP ROM

Some of the new AR9003 cards do not come with an external EEPROM chip
anymore. Calibration data on these cards is stored in the OTP ROM on
the chip.
This patch adds support for reading this data, and also adds support
for different EEPROM chip sizes (512 bytes instead of 1K).

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>