MAINTAINERS: remove Stefan Popa's email

The email is no longer active. This change removes Stefan's email from the
MAINTAINERS list and replaces it with Michael Hennerich's.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>