MythMusic: fix seek problem with restart playback.

When resuming after TV/video playback, the music track was resuming at the
start rather than seeking to the correct position.

#Fixes #12062.

Signed-off-by: Paul Harrison <pharrison@mythtv.org>
(cherry picked from commit 8af587a2c49d5411db0b46530f111e9aba85d72f)