drivers: flash: mcux_flexspi_nor: Remove flash reads while programming

Care must be taken to avoid any flash access while programming the flash
attached to the FlexSPI either via executing XIP code or reading RO data.

Remove locations where a constant device pointer might be dereferenced
within the mcux_flexspi_nor driver, to help avoid RWW hazards.

Fixes #64702

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>