ath5k: shifting the wrong variable for AR5K_AR5210

In the original code we shift "AR5K_PHY(256) >> 28" which is zero but
the intent was to shift the return value of ath5k_hw_reg_read() like we
do a couple lines later.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>