mailbox: Drop owner assignment from platform_driver

platform_driver does not need to set an owner because
platform_driver_register() will set it.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Lee Jones <lee@kernel.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>