mtd: spi-nor: micron-st.c: Replace hardcoded values for addr_nbytes/addr_mode_nbytes

We track in the core the internal address mode of the flash. Stop using
hardcoded values for the number of bytes of address and use
nor->addr_nbytes and nor->params->addr_mode_nbytes instead.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20220728041451.85559-3-tudor.ambarus@microchip.com