ath9k: remove unneeded calculation of minimal calibration power

Remove tMinCalPower from ath9k_hw_set_def_power_cal_table(), as it's
never used.  Remove corresponding arguments of the functions calculating
that value.

Original patch by Prarit Bhargava <prarit@redhat.com>

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