dfu: mmc: call fs functions instead of run_command

This unbreaks dfu mmc_file_op which is currently broken since using the
load cmd on a buffer from heap is not allowed - added with
commit aa3c609e ("fs: prevent overwriting reserved memory")

Fixes: commit aa3c609e ("fs: prevent overwriting reserved memory")
Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Lukasz Majewski <lukma@denx.de>