serial: 8250_exar: Remove useless has_slave member

Exar UARTs by default supports only up to 8 channels,
all above go as extension. Thus, there is no need to have
an additional property to distinguish them from first ones.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>