serial: exar: Add RS-485 support for Sealevel XR17V35X based cards

serial: exar: Add RS-485 support for Sealevel XR17V35X based cards

Sealevel XR17V35X based cards utilize DTR to control RS-485 Enable, but
the current implementation of 8250_exar uses RTS for the auto-RS485-Enable
mode of the XR17V35X UARTs. This patch implements DTR Auto-RS485 on
Sealevel cards.

Signed-off-by: Matthew Howell <matthew.howell@sealevel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenkoa@linux.intel.com>
Link: https://lore.kernel.org/r/4b8ad8ab6728742464c4e048fdeecb2b40522aef.camel@sealevel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>