drm/i915/xelpg: Fix write to MTL_MCR_SELECTOR

A misplaced closing parenthesis caused the groupid/instanceid values to
be considered part of the ternary operator's condition instead of being
OR'd into the resulting value.

Fixes: f32898c9 ("drm/i915/xelpg: Add multicast steering")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221019222437.3035182-1-matthew.d.roper@intel.com