ath9k: Remove unnecessary check

The commit "ath9k: Fix IQ calibration" added a check
to ensure that valid i2_p_q2_a0_d1 values are not discarded.
But since it is masked with 0xfff earlier, the codepath
will not be executed.

The earlier case where all values above 0x800 were considered
invalid is incorrect, since the HW can return valid values
between 0x800 and 0xfff.

Cc: Kai Shi <kaishi@qca.qualcomm.com>
Reported-by: Alex Hacker <hacker@epn.ru>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>