ath9k: simplify AR9220 fixup code for AR_AN_TOP2 register

Don't modify ah->iniModes, it's supposed to be constant.  Instead, apply
the fixup when the data is written to the registers.

Change ath9k_hw_init_eeprom_fix() to only determine whether the fixup is
needed.

This allows similteneous support for AR9220 cards that need AR_AN_TOP2
fixup (such as Ubiquiti SR71-12) and those that don't need it (D-Link
DWA-552 rev A2).

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>