usb: gadget: f_midi: missing unlock on error path

We added a new error path to this function and we forgot to drop the
lock.

Fixes: e1e3d7ec ('usb: gadget: f_midi: pre-allocate IN requests')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[mina86@mina86.com: rebased on top of refactoring commit]
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>

Signed-off-by: Felipe Balbi <balbi@kernel.org>