n64: use sector SECTOR_SHIFT instead 512

Instead of using magic numbers use SECTOR_SHIFT to get the number of
sectors from the size.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Lauri Kasanen <cand@gmx.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>