ath9k: remove eeprom txgain override for minor version < 19

ath9k_hw_4k_get_eeprom() overrides the eeprom value for txgain if the
minor version is not 19 or above with a value of 0.
ar9002_hw_init_mode_gain_regs() relies on this information to
determine whether this is a high power wifi card or not. The override
caused the driver to always use the 'normal' power tables even for
high power devices if their minor version was not high enough. Thus
leading to reduced power output.

This isn't needed for the AR9285; the check originated with the
AR9280 setup code which requires the EEPROM version check.

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Adrian Chadd <adrian@freebsd.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>