Fix MTE support from causing unused variable warnings

assert() calls are removed in release builds, and if that assert call is
the only use of a variable, an unused variable warning will be triggered
in a release build. This patch fixes this problem when
CTX_INCLUDE_MTE_REGS by not using an intermediate variable to store the
results of get_armv8_5_mte_support().

Change-Id: I529e10ec0b2c8650d2c3ab52c4f0cecc0b3a670e
Signed-off-by: Justin Chadwell <justin.chadwell@arm.com>

lib/el3_runtime/aarch64/context_mgmt.c

@@ -142,11 +142,11 @@ void cm_setup_context(cpu_context_t *ctx, const entry_point_info_t *ep)
 	 * world, and only for the secure world when CTX_INCLUDE_MTE_REGS is
 	 * set.
 	 */
-	unsigned int mte = get_armv8_5_mte_support();
 #if CTX_INCLUDE_MTE_REGS
-	assert(mte == MTE_IMPLEMENTED_ELX);
+	assert(get_armv8_5_mte_support() == MTE_IMPLEMENTED_ELX);
 	scr_el3 |= SCR_ATA_BIT;
 #else
+	unsigned int mte = get_armv8_5_mte_support();
 	if (mte == MTE_IMPLEMENTED_EL0) {
 		/*
 		 * Can enable MTE across both worlds as no MTE registers are