media: ov5640: adjust xclk_max

According to ov5640 datasheet xvclk is allowed to be between 6 and 54 MHz.
I run a successful test with 27 MHz.

Signed-off-by: Philipp Puschmann <pp@emlix.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>