arm64/esr: Use GENMASK() for the ISS mask

We express the mask for ESR_ELx.ISS in a non-standard manner, not using
the standard helpers. In preparation for adding decode for ISS2 convert to
use GENMASK(). No functional change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230417-arm64-iss2-dabt-decode-v3-1-c1fa503e503a@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>