mtd: spi-nor: spansion: switch h28hx's ready() to use vreg_offset

s28hx is the sole user of cypress_nor_set_octal_dtr, which already
uses vreg_offset to set octal DTR. Switch the ready method to use
vreg_offset as well. This is a preparation patch. The goal is to use
the same s28hx methods for the multi die version of the flash.

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Link: https://lore.kernel.org/r/20230726075257.12985-6-tudor.ambarus@linaro.org


Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>