dm: blk: Use IS_ENABLED() instead of #ifdefs in blk_post_probe

Use IS_ENABLED() instead of #ifdef in blk_post_probe function.

No functional change intended.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
[trini: Fix thinko and use CONFIG_HAVE_BLOCK_DEVICE in IS_ENABLED()]
Signed-off-by: Tom Rini <trini@konsulko.com>