mwifiex: do not advertise custom regulatory domain capability

mwifiex driver no longer supports it's own custom regulatory rules,
but custom regulatory domain capability is still advertised during
wiphy registration by the driver.

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>