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

arch/arm64/include/asm/esr.h

@@ -75,7 +75,7 @@
 
 #define ESR_ELx_IL_SHIFT	(25)
 #define ESR_ELx_IL		(UL(1) << ESR_ELx_IL_SHIFT)
-#define ESR_ELx_ISS_MASK	(ESR_ELx_IL - 1)
+#define ESR_ELx_ISS_MASK	(GENMASK(24, 0))
 #define ESR_ELx_ISS(esr)	((esr) & ESR_ELx_ISS_MASK)
 
 /* ISS field definitions shared by different classes */