brcmfmac: free primary net_device when brcmf_bus_start() fails

When initialization within brcmf_bus_start() fails on steps
before the brcmf_net_attach() the net_device for the primary
interface needs to be freed.

This patch resolves a panic during kernel boot as reported
by Stephen Warren.

ref.: http://mid.gmane.org/51AD1F22.2080004@wwwdotorg.org

Tested-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>