usb: gadget: mass_storage: set msg_registered after msg registered

If there is no UDC available, the msg register will fail and this
flag will not be set, but the driver is already added into pending
driver list, then the module removal modprobe -r can not remove
the driver from the pending list.

Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>