ALSA: core: Handle user defined ioctls

Handle ioctl magic 'U' as a user defined ioctl so that ALSA core calls
substream's ioctl handler.  This gets rid of a requirement that ALSA core
needs to be changed every time when a new user defined ioctl is introduced.
Change-Id: I390b9e0428bfadc302ff074417aa69163b12c6ec
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>
Signed-off-by: Meng Wang <mwang@codeaurora.org>