usb-gadget/tcm: Convert to TARGET_SCF_ACK_KREF I/O krefs

This patch drops struct usbg_cmd->kref internal kref-erence
usage, for proper TARGET_SCF_ACK_KREF conversion.

Tested-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>