brcmfmac: Change error print on wlan0 existence.

During initialization of the device, but also on some other
moments the driver prints an error that the netdev already exists.
This is a result of the way the driver is initializing the
firmware and not really an error. The code is not treating it as
an error either. This error print has resulted in many questions
by users and is confusing and incorrect. This patch changes the
error log into a debug info log.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>