media: av7110_v4l: avoid a typecast

While smatch reports an issue there:

	drivers/media/pci/ttpci/av7110_v4l.c:163 ves1820_set_tv_freq() warn: unsigned 'freq' is never less than zero.
	drivers/media/pci/ttpci/av7110_v4l.c:165 ves1820_set_tv_freq() warn: unsigned 'freq' is never less than zero.

The logic is actually fine. Yet, removing the typecast
shuts up smatch and makes the code more readable.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>