tile: cleanup the comment in init_pgprot

In tile vmlinux, the rodata area start after the _sdata.
The rodata area is included between [_sdata, __end_rodata),
and is handled at an earlier point.
The page walk starts at __end_rodata, not _sdata.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>