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