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>