mtd: spi-nor-core: Default to addr_width of 3 for configurable widths

JESD216D-01 mentions that "defaults to 3-Byte mode; enters 4-Byte mode on
command."

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>