net: pfe_eth: Remove non-DM code check from pfe_spi_flash_init

CONFIG_DM_SPI_FLASH is only supported now with passing of driver
conversion deadline from non-DM to DM model. Hence, it's safe to remove
non-DM code check from pfe_spi_flash_init.

Also use CONFIG_ENV_SPI_MODE and CONFIG_ENV_SPI_MAX_HZ instead of
reading reading values from DT.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>