mtd: spi-nor: Use CONFIG_IS_ENABLED for CONFIG_SPI_FLASH_BAR defines

mtd: spi-nor: Use CONFIG_IS_ENABLED for CONFIG_SPI_FLASH_BAR defines

At the moment a mixture of ifdef(CONFIG_IS_ENABLED) and
CONFIG_IS_ENABLED(SPI_FLASH_BAR) is used in the spi-nor framework.
This leads to misbehaviour in the SPL as there is no Kconfig option
CONFIG_SPL_SPI_FLASH_BAR. This commit standardizes the use of
CONFIG_SPI_FLASH to get SPLs that load U-Boot proper from the
SPI flash to work again.

Fixes: 9bb02f7 (mtd: spi-nor: Fix the spi_nor_read() when config SPI_STACKED_PARALLEL is enabled)
Signed-off-by: Bernhard Messerklinger <bernhard.messerklinger@br-automation.com>