mtd: spi-nor-core: Rework spansion_read/write_any_reg() to use addr_mode_nbytes

Read/Write Any Register commands take 3- or 4- byte address depending on
flash's internal address mode. The nor->addr_width tracks number of
address bytes used in read/program/erase ops that can be 4
(with 4B opcodes) regardless of flash's internal address mode. The
nor->addr_mode_nbytes tracks flash's internal address mode so replace
nor->addr_width by that.

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