drm/i915: simplify gmbus xfer error checks

Shorter, easier to follow code with no functional changes. In all cases,
the return value ultimately comes from gmbus_wait_hw_status() anyway.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448980166-23055-1-git-send-email-jani.nikula@intel.com