usb: gadget: f_midi: check kstrdup() return value

kstrdup() might fail, so check its return value and react appropriately.

BACKPORT FROM MAINLINE KERNEL

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Change-Id: Ife54cb138b51fd84ac2b62aa340747789894a320