My network card : iwn0: <Intel(R) Centrino(R) Ultimate-N 6300> mem 0xd3300000-0xd3301fff irq 19 at device 0.0 on pci68

/etc/rc.conf contains the following:
wlans_iwn0="wlan0"
ifconfig_wlan0="WPA DHCP"

And /etc/wpa_supplicant.conf has the appropriate settings for some access points :
network={
	ssid="myssid"
	psk=myasciikey
}

When the system boots or when I start the interface with netif, I have a fatal firmware error and I can't use my card. I tried to disable the bgscan but it didn't change anything :

wlan0: Ethernet address: 00:24:d7:XX:XX:XX
iwn0: iwn_intr: fatal firmware error
firmware error log:
  error type      = "SYSASSERT" (0x00000005)
  program counter = 0x00003C2C
  source line     = 0x00000580
  error data      = 0x0000000100000000
  branch link     = 0x00003C2800003C28
  interrupt link  = 0x0000153200000000
  time            = 183809626
driver status:
  tx ring  0: qid=0  cur=0   queued=0  
  tx ring  1: qid=1  cur=0   queued=0  
  tx ring  2: qid=2  cur=0   queued=0  
  tx ring  3: qid=3  cur=2   queued=0  
  tx ring  4: qid=4  cur=64  queued=0  
  tx ring  5: qid=5  cur=0   queued=0  
  tx ring  6: qid=6  cur=0   queued=0  
  tx ring  7: qid=7  cur=0   queued=0  
  tx ring  8: qid=8  cur=0   queued=0  
  tx ring  9: qid=9  cur=0   queued=0  
  tx ring 10: qid=10 cur=0   queued=0  
  tx ring 11: qid=11 cur=0   queued=0  
  tx ring 12: qid=12 cur=0   queued=0  
  tx ring 13: qid=13 cur=0   queued=0  
  tx ring 14: qid=14 cur=0   queued=0  
  tx ring 15: qid=15 cur=0   queued=0  
  tx ring 16: qid=16 cur=0   queued=0  
  tx ring 17: qid=17 cur=0   queued=0  
  tx ring 18: qid=18 cur=0   queued=0  
  tx ring 19: qid=19 cur=0   queued=0  
  rx ring: cur=14

the link changes from DOWN to UP to DOWN.

I have seen that after every netif start I have two wpa_supplicant process ?

How-To-Repeat: At every boot or netif start