MTD: nandsim: suppress unnecessary warning

nand_base sometimes reads only 2 bytes of a 4 byte id.
It is OK.  Do not print a warning in that case.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>