media: mmp: Bring back registration of the device

In commit 4af65141 ("media: marvell: cafe: Register V4L2 device
earlier"), a call to v4l2_device_register() was moved away from
mccic_register() into its caller, marvell/cafe's cafe_pci_probe().
This is not the only caller though -- there's also marvell/mmp.

Add v4l2_device_register() into mmpcam_probe() to unbreak the MMP camera
driver, in a fashion analogous to what's been done to the Cafe driver.
Same for the teardown path.

Fixes: 4af65141 ("media: marvell: cafe: Register V4L2 device earlier")
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: stable@vger.kernel.org # v6.6+
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>