media: mxl111sf: improve error handling logic

As reported by smatch:
	drivers/media/usb/dvb-usb-v2/mxl111sf-demod.c:485 mxl111sf_demod_read_signal_strength() error: uninitialized symbol 'modulation'.

The mxl111sf_demod_read_signal_strength() just ignores if something
gets wrong while reading snr or modulation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>