mmc: mmc_test: Only warn about not waiting for busy if it's supported

If the MMC host controller does not support waiting for card signaling
busy state (MMC_CAP_WAIT_WHILE_BUSY cap), there is no point in prining
the relevant warning message.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Reviewed-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Chris Ball <cjb@laptop.org>