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

Fixes: 8b8b3d6b ("stm32mp1: board: add environment variable for board id and board rev")
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: NPatrice Chotard <patrice.chotard@st.com>