crda: fix check-package warning

In commit 3e79bd3f ("crda: bump
version to 4.14"), the value of CRDA_SOURCE was changed to use what is
the default value for <pkg>_SOURCE, triggering a warning from
check-package:

package/crda/crda.mk:8: remove default value of _SOURCE variable (http://nightly.buildroot.org/#generic-package-reference

)

This commit fixes that by dropping CRDA_SOURCE entirely.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>