MAINTAINERS: update for mwifiex driver maintainers

Ganapathi & Xinming are starting to take a more active role in the
mwifiex driver maintainership here onwards on account of organizational
changes.

CC: Xinming Hu <huxm@marvell.com>
CC: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>