tzc380: Fix some asserts

This driver can be compiled in release builds, but GCC generates warnings
for some comparisons and that prevents the firmware from being built in
debug builds.

Change-Id: Ic52e1b4a11896ecf086864fbe2b5bfc143ec9b1b
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>