wtv: Make WTV_SECTOR_BITS a 64 bit constant

This makes sure that values that are left-shifted by this constant
end up casted to 64 bit before shifting, avoiding overflow if the
value ends up larger than 2 GB.

Signed-off-by: Martin Storsjö <martin@martin.st>
wtv: Drop some casts that now are unnecessary

Signed-off-by: Martin Storsjö <martin@martin.st>