misdn: avoid -Wempty-body warning

gcc warns about a pointless condition:

drivers/isdn/hardware/mISDN/hfcmulti.c: In function 'hfcmulti_interrupt':
drivers/isdn/hardware/mISDN/hfcmulti.c:2752:17: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
 2752 |                 ; /* external IRQ */

As the check has no effect, just remove it.
Suggested-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/hardware/mISDN/hfcmulti.c

@@ -2748,8 +2748,6 @@ hfcmulti_interrupt(int intno, void *dev_id)
 		if (hc->ctype != HFC_TYPE_E1)
 			ph_state_irq(hc, r_irq_statech);
 	}
-	if (status & V_EXT_IRQSTA)
-		; /* external IRQ */
 	if (status & V_LOST_STA) {
 		/* LOST IRQ */
 		HFC_outb(hc, R_INC_RES_FIFO, V_RES_LOST); /* clear irq! */