usb/gadget: f_mass_storage: use string literal as format in dev_set_name

Fix commit b27c08c9

 where dev_set_name()
is used without a string literal as format.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>