MAINTAINERS: Update Microchip MCP3911 to Maintained

The actual status of the code is Maintained.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Cc: Kent Gustavsson <kent@minoris.se>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio <linux-iio@vger.kernel.org>
Link: https://lore.kernel.org/r/20220723092030.260812-1-marcus.folkesson@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>