serial: imx: Correct comment imx_flush_buffer()

The comment in imx_flush_buffer() states that the state of 4 registers
are to be saved/restored, then only saves and restores 3 registers. The
missing register (UBRC) is read only and thus can't be restored.

Update the comment to reflect reality.

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>