usb: gadget: f_tcm: out of bound access in usbg_drop_tpg

Commit dc8c46a5 ("usb: gadget: f_tcm: convert to new function
interface with backward compatibility") introduced a possible out
of bounds memory access:

If tpg is not found in function usbg_drop_tpg,
tpg_instances[TPG_INSTANCES] is accessed.

Fixes: dc8c46a5 ("usb: gadget: f_tcm: convert to new function interface with backward compatibility")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>