brcmfmac: remove error message upon allocation failure

In function brcmf_add_if() an error message is printed
upon alloc_netdev() failure. The allocation failure itself
spews enough info in the log so remove the error message.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>