Unfortunately, no OpenBSD driver for the Broadcom BCM4313 exists. The bwfm driver support Broadcom "FullMAC" chips but the BCM4313 is a "SoftMAC" chip and would need a different driver.
There is a driver for this chip in Linux (brcmsmac) so it might be possible for someone to write an OpenBSD driver but this is no small undertaking. Your best bet may be to replace the onboard wireless with a card that is supported by OpenBSD. - todd
