usb: gadget: audio: Prevent opening playback and capture stream twice

Voice over USB mode, playback and capture streams are opened twice from
two threads at the same time, when playback and capture devices are
selected together in host PC. This would cause memory map failure with AFE.
Fix this by splitting the gaudio_open_streams() function into
gaudio_open_playback_streams() and gaudio_open_capture_streams().

CRs-Fixed: 883629
Change-Id: I388b13558c3445c3d7e4694b9e4ed6fdd33596a1
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>