media: ddbridge: fix impossible condition warning

Smatch and gcc complained:

  drivers/media/pci/ddbridge/ddbridge-core.c:3491 bpsnr_show() warn: impossible condition '(snr[0] == 255) => ((-128)-127 == 255)'

  drivers/media/pci/ddbridge/ddbridge-core.c: In function ‘bpsnr_show’:
  drivers/media/pci/ddbridge/ddbridge-core.c:3491:13: warning: comparison is always false due to limited range of data type [-Wtype-limits]

Fix this by changing the type of snr to unsigned char.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Tested-by: Jasmin Jessich <jasmin@anw.at>
Tested-by: Dietmar Spingler <d_spingler@freenet.de>
Tested-by: Manfred Knick <Manfred.Knick@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>