tty: serial: imx: Add fast path when rs485 delays are 0

Right now, even when `delay_rts_before_send` and `delay_rts_after_send`
are 0, the hrtimer is triggered (with timeout 0) which can introduce a
few 100us of additional overhead on slower i.MX platforms.

Implement a fast path when the delays are 0, where the RTS signal is
toggled immediately instead of going through an hrtimer.  This fast path
behaves identical to the code before delay support was implemented.

Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Harald Seiler <hws@denx.de>
Link: https://lore.kernel.org/r/20220119145204.238767-1-hws@denx.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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