mwifiex: retrieve correct max_power information in reg_notifier handler

As we don't provide custom regulatory rules to cfg80211,
"chan->max_power" remains uninitialized (0dbm) and
"chan->max_reg_power" will contain maximum power for a channel
extracted from regulatory rules provided by CRDA; hence use
"chan->max_reg_power" in reg_notifier handler instead of
"chan->max_power" to set max_power in firmware.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>