mmc: wbsd: Silence compiler warning

The commit "mmc: wbsd: Move away from using deprecated APIs" introduced
a compiler warning, let's silence it.

Cc: Pierre Ossman <pierre@ossman.eu>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>