sc16is7xx: remove RS-485 delay RTS handling

Users of RS-485 can request via ioctl that RTS signals should
be activated selected number of milliseconds before the actual
data transmission or delay reception certain number of milli-
seconds after the transmission is finished.  In sc16is7xx,
however, RTS signalling is handled by the hardware and driver
has no way of providing this feature.

We still try to provide .delay_rts_before_send by delaying
transmission but without actual effect on the RTS line.

Note: this change will make the driver return -EINVAL when the
      feature is requested (.delay_rts_after_send is set).

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>