media: ar0521: don't overflow when checking PLL values

The PLL checks are comparing 64 bit integers with 32 bit
ones, as reported by Coverity. Depending on the values of
the variables, this may underflow.

Fix it ensuring that both sides of the expression are u64.

Fixes: 852b50ae ("media: On Semi AR0521 sensor driver")
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>