brcmfmac: add dedicated log level for low-level sdio debugging

The low-level sdio code has a large number of trace and info messages
that are mostly useful looking into bus specific issues. For tracing
higher-level driver functions it is better to have a dedicated level
for low-level sdio debugging.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Change-Id: Ia424ff18d9033b97aeffc248358e50c51805e815
Reviewed-on: http://lb-bun-88.bun.broadcom.com:8080/74


Signed-off-by: John W. Linville <linville@tuxdriver.com>