board: st: common: update test on misc_read result in command stboard

Update management of misc_read/misc_write, which now returns length of
data after the commit 8729b1ae ("misc: Update read() and write()
methods to return bytes xfered"): raise a error when the result is not
the expected length.

Fixes: 658fde8a ("board: stm32mp1: stboard: lock the OTP after programming")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>