media: ov08x40: Rename ext_clk to xvclk

The data-sheet and documentation for this part uses the name xvclk not
ext_clk for the input reference clock. Rename the variables and defines in
this driver to align with the data-sheet name.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>