A line in the upstream SPDs is only compiled in in `DEBUG` builds. This
line is used to help with assertions and so assertion failures can
happen in release builds with assertions enabled. Use
`ENABLE_ASSERTIONS` instead of `DEBUG`.

This bug was introduced in commit aa61368e

, which introduced the build
option `ENABLE_ASSERTIONS`.

Change-Id: I7977df9c89c68677b00099b2a1926fa3cb0937c6
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>