serial: exar: Fix feature control register constants

According to the XR17V352 manual, bit 4 is IrDA control and bit 5 for
485. Fortunately, no driver used them so far.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>