USB: gadget: audio driver seg-fault fix

The included patch can be applied to the new usb gadget audio driver.

It addresses a seg-fault in uncovered in g_audio.ko.
The fault occurs in the function u_audio.c::gaudio_open_end_dev() when
device /dev/snd/pcmC0D0c (FILE_PCM_CAPTURE) is not present.

I suspect there may be similar problems with device /dev/snd/pcmC0D0p
(FILE_PCM_PLAYBACK) handling also.  I leave that for the developer(s),
as I was unsure as to the side-effects of not calling
playback_default_hw_params() in the initialization phase.

Signed-off-by: Robin Callender <robin_callender@hotmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>