checkpatch.pl: Make CONFIG_IS_ENABLED(CONFIG_*) an error

CONFIG_IS_ENABLED() takes the kconfig name without the CONFIG_ prefix,
e.g. CONFIG_IS_ENABLED(CLK) for CONFIG_CLK. Make including the prefix
an error in checkpatch.pl so calls in the wrong format aren't
accidentally reintroduced.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>