sparc: use symbolic names for tsb indexing

Use symbolic names MM_TSB_BASE and MM_TSB_HUGE instead of numeric values
0 and 1 in __tsb_context_switch.  Code cleanup only, no functional change.

Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>