NPCX: Correct ESPICFG fields

The parameters to FIELD() should be the position and size of each bit
field.  Correct the size of IOMODE and MAXFREQ.  Otherwise, the MAXFREQ
field is overwritten by the IOMODE update during espi_npcx_configure().

Signed-off-by: Diana Zigterman <dzigterman@google.com>