msm: usbaudio: Add support for USB headset detection

 - During bootup, create a file system storing the
   information about usb device state and name, and
   broadcast the message to userspace whenever the
   device is plugged or unplugged.

 - Register the device at USB sound card initialization
   for creating file system and toggle the state in that
   filesystem when USB is plugged/unplugged. This will
   also send a message to user space informing it about
   state change.

Change-Id: Icd78273bb765a26bbc70725afebe8955de8f7315
Signed-off-by: Harmandeep Singh <hsingh@codeaurora.org>