ath9k: never read from the AR_IMR_S2 register

The AR_IMR_S2 register sometimes cannot be read correctly.  Instead of a
valid value, 0xdeadbeef is returned.  The driver has been observed
writing that value back to AR_IMR_S2 after changing a few bits.

Cache the register value in ah->imrs2_reg and always write chached value
to the register.

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