serial: exar: Enable MSI support

Use pci_alloc_irq_vectors to enable MSI when available. At least the
XR17V352 supports this.

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>