usb: gadget: f_tcm: Provide support to get alternate setting in tcm function

Providing tcm_get_alt in tcm function to support Bulk only protocol and
USB Attached SCSI protocol

Signed-off-by: Jayshri Pawar <jpawar@cadence.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>