pcm: rewindable, forwardable: don't return stale data

pcm: rewindable, forwardable: don't return stale data

The current behavior of snd_pcm_rewindable and snd_pcm_forwardable means
that the returned value is only accurate to one period. Or maybe even
meaningless if period interrupts are off. Fetch the up-to-date position
of the hardware pointer, as that's what is wanted by callers.

Signed-off-by: Alexander E. Patrakov <patrakov@gmail.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>