arm64: Update comment for ASID() macro

Commit 25b92693 ("arm64: mm: convert cpu_do_switch_mm() to C") added
a new use of the ASID() macro, so update the comment in asm/mmu.h which
reasons about why an atomic reload of 'mm->context.id.counter' is not
required.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>