board: stm32mp1: update command stboard on misc_write result

Update management of misc_write, which now return length of data
after the commit 8729b1ae ("misc: Update read() and write()
methods to return bytes xfered")

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>