arm64: proton-pack: Update comment to reflect new function name

The function detect_harden_bp_fw() is gone after commit d4647f0a
("arm64: Rewrite Spectre-v2 mitigation code"). Update this comment to
reflect the new state of affairs.

Fixes: d4647f0a

 ("arm64: Rewrite Spectre-v2 mitigation code")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Cc: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201020214544.3206838-3-swboyd@chromium.org
Signed-off-by: Will Deacon <will@kernel.org>