mwifiex: do not advertise custom regulatory domain capability

Since we don't support custom regulatory domains,
WIPHY_FLAG_CUSTOM_REGULATORY should not be enabled during wiphy
registration.

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>