mtd: spi: Use CONFIG_IS_ENABLED to prevent ifdef

Use CONFIG_IS_ENABLED to prevent ifdef in sf_probe.c

Cc: Simon Glass <sjg@chromium.org>
Cc: Vignesh R <vigneshr@ti.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>