USB Gadget file_storage.c: remove unnecessary casts

Went looking through some usb stuff and found some unnecessary casts in
file_storage.c  This is part of the KernelJanitors TODO list.

Signed-off-by: John Daiker <daikerjohn@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>