powerpc/83xx/mpc832x_rdb: call platform_device_put() in error case in of_fsl_spi_probe()

If platform_device_add() is not called or failed, it can not call
platform_device_del() to clean up memory, it should call
platform_device_put() in error case.

Fixes: 26f6cb99

 ("[POWERPC] fsl_soc: add support for fsl_spi")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221029111626.429971-1-yangyingliang@huawei.com