drm/i915/hti: avoid theoretically possible negative shift

If phy is PHY_NONE, the shift to register bits becomes negative. Check
and warn about this.

Reported-by: coverity-bot <keescook@chromium.org>
References: https://lore.kernel.org/r/202211180848.D39006C@keescook


Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221122120948.3436180-1-jani.nikula@intel.com