media: marvell: Simplify remove callback

Fold mmpcam_remove() into its only caller mmpcam_platform_remove(). Note
that cam can never be zero, as the probe function calls
platform_set_drvdata() with a non-NULL argument (or returns an error
code in which case .remove() won't be called).

Also use mmpcam_remove() as the function name for the remove callback to
align to mmpcam_probe() for .probe().

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>