MAINTAINERS: update for mwifiex driver maintainers

Amitkumar and Avinash are taking care of mwifiex driver as the
maintainers now due to organizational change.

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