usb: gadget: inode.c: fix unbalanced spin_lock in ep0_write

Fix bad unlock balance: ep0_write enter with the locks locked from
inode.c:1769, hence it must exit with spinlock held to avoid double
unlock in dev_config.

Signed-off-by: David Eccher <d.eccher@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>