usb: gadget: ffs: Make sparse happier

Silences the following warnings:
drivers/usb/gadget/function/f_fs.c:1253:37: warning: incorrect type in argument 1 (different address spaces)
drivers/usb/gadget/function/f_fs.c:1253:37:    expected void [noderef] <asn:1>*to
drivers/usb/gadget/function/f_fs.c:1253:37:    got void *<noident>
drivers/usb/gadget/function/f_fs.c:2322:23: warning: cast to restricted __le32
drivers/usb/gadget/function/f_fs.c:2876:38: warning: cast to restricted __le32
drivers/usb/gadget/function/f_fs.c:272:12: warning: context imbalance in '__ffs_ep0_queue_wait' - unexpected unlock
drivers/usb/gadget/function/f_fs.c:450:17: warning: context imbalance in 'ffs_ep0_write' - different lock contexts for basic block
drivers/usb/gadget/function/f_fs.c:490:24: warning: context imbalance in '__ffs_ep0_read_events' - unexpected unlock
drivers/usb/gadget/function/f_fs.c:496:16: warning: context imbalance in 'ffs_ep0_read' - different lock contexts for basic block

Also, add an "unlocks spinlock" comment for consistency with existing ones.
No behaviour change is intended.

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>