media: ov6650: Fix arbitrary selection of master clock rate

A hardcoded 12 MHz master clock frequency has been assumed since
conversion of the driver from soc_camera sensor to a standalone V4L2
subdevice by commit 23a52386 ("media: ov6650: convert to standalone
v4l2 subdevice").  Fix it.

Define a static table of supported master clock rates (fix misnamed
symbol while being at it), then use v4l2_clk_get/set_rate() to obtain
a clock rate matching one of those supported.  On success, apply
respective master clock hardware divisor provided by the matching
element of the table.

[Sakari Ailus: Initialize xclk to NULL.]

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>