mtd: spi-nor: spansion: switch set_octal_dtr method to use vreg_offset

mtd: spi-nor: spansion: switch set_octal_dtr method to use vreg_offset

All the Infineon flashes that currently support octal DTR mode
define the optional SCCR SFDP table, thus all retrieve vreg_offset.
Switch all the available octal DTR Infineon flashes to use the
volatile register offset to set the configuration registers. The goal
is to have a single pair of methods for both single/multi-chip package
devices.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Link: https://lore.kernel.org/r/20230726075257.12985-5-tudor.ambarus@linaro.org
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>