serial: imx: drop if that always evaluates to true

The check sts & USR1_DTRD was just evaluated to true two lines above.
So this change doesn't have any effect on the semantic of the driver.

Fixes: 27e16501 ("serial: imx: implement DSR irq handling for DTE mode")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>