dfu: mmc: call fs functions instead of run_command

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 aa3c609e2be5 ("fs: prevent overwriting reserved memory")

Fixes: commit aa3c609e2be5 ("fs: prevent overwriting reserved memory")
Reported-by: Stephen Warren <[email protected]>
Signed-off-by: Simon Goldschmidt <[email protected]>
Tested-by: Stephen Warren <[email protected]>
Acked-by: Lukasz Majewski <[email protected]>