usb: gadget: f_subset: Fix unbalanced refcnt in geth_free

usb: gadget: f_subset: Fix unbalanced refcnt in geth_free

geth_alloc() increments the reference count, but geth_free() fails to
decrement it. This prevents the configuration of attributes via configfs
after unlinking the function.

Decrement the reference count in geth_free() to ensure proper cleanup.

Fixes: 02832e56f88a ("usb: gadget: f_subset: add configfs support")
Cc: stable@vger.kernel.org
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://patch.msgid.link/20260320-usb-net-lifecycle-v1-1-4886b578161b@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>