mmc: mmc_test: remove BUG_ONs and deploy error handling

It is unnecessary to panic the kernel when testing mmc. Instead,
cast a warning for folkz to debug and return the error code to
the caller to indicate the failure of this test should be enough.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>