sunvdc: Balance device refcount in vdc_port_mpgroup_check

sunvdc: Balance device refcount in vdc_port_mpgroup_check

Using device_find_child() to locate a probed virtual-device-port node
causes a device refcount imbalance, as device_find_child() internally
calls get_device() to increment the device’s reference count before
returning its pointer. vdc_port_mpgroup_check() directly returns true
upon finding a matching device without releasing the reference via
put_device(). We should call put_device() to decrement refcount.

As comment of device_find_child() says, 'NOTE: you will need to drop
the reference with put_device() after use'.

Found by code review.

Cc: stable@vger.kernel.org
Fixes: 3ee70591d6c4 ("sunvdc: prevent sunvdc panic when mpgroup disk added to guest domain")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Link: https://lore.kernel.org/r/20250719075856.3447953-1-make24@iscas.ac.cn
Signed-off-by: Jens Axboe <axboe@kernel.dk>