fmcomms2: Don't crash if gain mode list is not populated

Check whether we got a valid string from the gain mode list control before
passing it to strcmp(). Fixes a crash in forced plugin mode.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Check 2nd gain control mode when updating the gain for the 2nd channel

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Print error if we can't find object

Print an error if we can not find the object in g_builder_connect_signal().

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>