modem.conf Off-hook improve behavior

modem.conf Off-hook improve behavior

Only restore the old value if it differs from the requested
value, because if it has changed restoring the old value
overrides the change.  Take for example, a voice modem with
a .conf that sets preserve off-hook.  Start playback (on-hook
to off-hook), start record (off-hook to off-hook), stop
playback (off-hook to restore on-hook), stop record (on-hook
to restore off-hook), Clearly you don't want to leave the
modem "on the phone" now that there isn't any playback or
recording active.

Signed-off-by: David Fries <david@fries.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>