media: rcar-csi2: Rename confirm_start() to phy_post_init() to match its usage

Since the driver was picked-up the starting of the PHY have changed
quiet a bit. An artifact of these changes is the now poorly named
callback confirm_start(). It used to confirm start of the PHY but now
performs post PHY start initialization, rename it to phy_post_init() to
reflect this.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>