busses/mmc/sdhci_pci: Return B_UNHANDLED_INTERRUPT at end of non-void func

* I'm making an assumption here this is the desired result since interrupts
  are unhandled at the end of the function.
* Pointed out by gcc

src/add-ons/kernel/busses/mmc/sdhci_pci.cpp

@@ -358,6 +358,7 @@ sdhci_generic_interrupt(void* data)
 	intmask &= ~(SDHCI_INT_BUS_POWER | SDHCI_INT_CARD_INS
 		| SDHCI_INT_CARD_REM | SDHCI_INT_CMD_MASK);
 
+	return B_UNHANDLED_INTERRUPT;
 }