fix(bl31): fix validate_el3_interrupt_rm preprocessor usage

Fix the "#if defined(FOO)" usage introduced by commit 7c2fe62f
("fix(bl31): allow use of EHF with S-EL2 SPMC") since the defines are
always passed as -DFOO=0 or as -DFOO=1. The "#if defined(FOO)" will now
always be true which is wrong.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: I84fb144debc9899727a1fc021acdd59b4a6f0171