cputlb: Make tlb_n_entries private to cputlb.c

cputlb: Make tlb_n_entries private to cputlb.c

There are no users of this function outside cputlb.c,
and its interface will change in the next patch.

Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>