mmc: block: propagate correct returned value in mmc_rpmb_ioctl

In commit 97548575

 ("mmc: block: Convert RPMB to a character device") a
new function `mmc_rpmb_ioctl` was added. The final return is simply
returning a value of `0` instead of propagating the correct return code.

Discovered during a compilation with W=1, silence the following gcc warning

drivers/mmc/core/block.c:2470:6: warning: variable ‘ret’ set but not used
[-Wunused-but-set-variable]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Fixes: 97548575

 ("mmc: block: Convert RPMB to a character device")
Cc: stable@vger.kernel.org # v4.15+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>