mtip32xx: fix shift larger than type warning

If we're building a 32-bit kernel and CONFIG_LBADF isn't set,
sector_t is 32-bits wide. The shifts by 32 and 40 are thus
larger than we support.

Cast the sector offset to a u64 to avoid these warnings.

Signed-off-by: Jens Axboe <axboe@kernel.dk>