arm64: assembler: update stale comment for disable_step_tsk

A comment in the disable_step_tsk macro refers to synchronising with
enable_dbg, as historically the entry used enable_dbg to unmask debug
exceptions after disabling single-stepping.

These days the unmasking happens in entry-common.c via
local_daif_restore() or local_daif_inherit(), so the comment is stale.
This logic is likely to chang in future, so it would be best to avoid
referring to those macros specifically.

Update the comment to take this into account, and describe it in terms
of clearing DAIF.D so that it doesn't macro where this logic lives nor
what it is called.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240422113523.4070414-2-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/assembler.h

@@ -59,7 +59,7 @@
 	mrs	\tmp, mdscr_el1
 	bic	\tmp, \tmp, #DBG_MDSCR_SS
 	msr	mdscr_el1, \tmp
-	isb	// Synchronise with enable_dbg
+	isb	// Take effect before a subsequent clear of DAIF.D
 9990:
 	.endm