media: ov5640: Fix check for PLL1 exceeding max allowed rate

The variable _rate is by ov5640_compute_sys_clk() which returns
zero if the PLL exceeds 1GHz.  Unfortunately, the check to see
if the max PLL1 output is checking 'rate' and not '_rate' and
'rate' does not ever appear to be 0.

This patch changes the check against the returned value of
'_rate' to determine if the PLL1 output exceeds 1GHz.

Fixes: aa288248 ("media: ov5640: Adjust the clock based on the expected rate")
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>