brcmfmac: fix uninitialized warning in brcmf_usb_probe_phase2()

This fixes the following warning:

  drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c: In function
  'brcmf_usb_probe_phase2':
  drivers/net/wireless/broadcom/brcm80211/brcmfmac/usb.c:1198:2:
  warning: 'devinfo' may be used uninitialized in this function
  [-Wmaybe-uninitialized]
    mutex_unlock(&devinfo->dev_init_lock);

Fixes: 6d0507a7 ("brcmfmac: add parameter to pass error code in firmware callback")
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>