usb: gadget: f_uac2: disable IN/OUT ep if unused

Via p_chmask/c_chmask the user can define whether uac2 shall support
playback and/or capture. This has only effect on the created ALSA device,
but not on the USB descriptor. This patch adds playback/capture descriptors
dependent on that parameter.
Signed-off-by: Andreas Pape <apape@de.adit-jv.com>
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>