linux/serdev.h: Replace 'ctrl->serdev' with 'serdev'

Replace 'ctrl->serdev' with 'serdev' in serdev_controller_write_wakeup()
and serdev_controller_receive_buf().

Cc: Rob Herring <robh@kernel.org>
Cc: cphealy@gmail.com
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>