media: saa7134: avoid a shift overflow

As reported by smatch:
	drivers/media/pci/saa7134//saa7134-tvaudio.c:686 saa_dsp_writel() warn: should 'reg << 2' be a 64 bit type?

On a 64-bits Kernel, the shift might be bigger than 32 bits.

In real, this should never happen, but let's shut up the warning.

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