usb: gadget: f_uac1: free audio device card on USB disconnect

Driver opens audio sound device card on USB connect and
does not close on disconnect. When next time connect happens
driver checking for sound device card and returns if already
exist and causing issue on next connect. Hence fix this issue
by freeing the device card during disconnect.

Change-Id: I46f9a353f73b6d4f2297ad064f352f50f293a1d0
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>