serdev: make synchronous write helper interruptible

Allow the synchronous serdev_device_write() helper to be interrupted.

This is useful for cases where I/O is performed on behalf of user space
and we don't want to block indefinitely when using flow control.

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>