fix irqstats

my `/proc/interupts` looks like the following:

               CPU0       
      3:   10494710   ARMCTRL  BCM2708 Timer Tick
     32:   75437760   ARMCTRL  dwc_otg, dwc_otg_pcd, dwc_otg_hcd:usb1
     52:          0   ARMCTRL  BCM2708 GPIO catchall handler
     65:     461242   ARMCTRL  ARM Mailbox IRQ
     66:      33844   ARMCTRL  VCHIQ doorbell
     75:          1   ARMCTRL
     77:    2412182   ARMCTRL  bcm2708_sdhci (dma)
     83:         19   ARMCTRL  uart-pl011
     84:    3222800   ARMCTRL  mmc0
    FIQ:              usb_fiq
    Err:          0


In the `FIQ` row there is no number so i get these warning messages from `munin-node.log`:

    2013/12/07-06:28:55 [4973] Error output from irqstats:
    2013/12/07-06:28:55 [4973]      Argument "usb_fiq" isn't numeric in addition (+) at /etc/munin/plugins/irqstats line 95, <$in> line 11.

This pull request fixes these warnings