staging: gpib: Fix device reference leak in fmh_gpib driver

The fmh_gpib driver contains a device reference count leak in
fmh_gpib_attach_impl() where driver_find_device() increases the
reference count of the device by get_device() when matching but this
reference is not properly decreased. Add put_device() in
fmh_gpib_detach(), which ensures that the reference count of the
device is correctly managed.

Found by code review.

Cc: stable <stable@kernel.org>
Fixes: 8e4841a0 ("staging: gpib: Add Frank Mori Hess FPGA PCI GPIB driver")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>