serial: 8250_exar: Drop unused variable in pci_xr17v35x_setup()

After removal has_slave use the variable board becomes unused.
Remove it to avoid a warning:

drivers/tty/serial/8250/8250_exar.c: In function 'pci_xr17v35x_setup':
drivers/tty/serial/8250/8250_exar.c:380:31: warning: unused variable 'board' [-Wunused-variable]
  const struct exar8250_board *board = priv->board;
                               ^~~~~

Fixes: 328c11f24b57 ("serial: 8250_exar: Remove useless has_slave member")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_exar.c

@@ -377,7 +377,6 @@ static int
 pci_xr17v35x_setup(struct exar8250 *priv, struct pci_dev *pcidev,
 		   struct uart_8250_port *port, int idx)
 {
-	const struct exar8250_board *board = priv->board;
 	const struct exar8250_platform *platform;
 	const struct dmi_system_id *dmi_match;
 	unsigned int offset = idx * 0x400;