selftests/arm64: Use switch statements in mte_common_util.c

In the MTE tests there are several places where we use chains of if
statements to open code what could be written as switch statements, move
over to switch statements to make the idiom clearer.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220510164520.768783-6-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>