media: dvb-frontends/stv0910: fix STR assignment, remove unneeded var

According to the documentation, FE_SCALE_DECIBEL values should be assigned
to .svalue and not .uvalue, so let's do this. While at it, remove the
unneeded strength var from read_signal_strength().

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>