mlx5: prevent 64bit divide

mlx5_tout_ms() returns a u64, we can't directly divide it.
This is not a problem here, @timeout which is the value
that actually matters here is already a ulong, so this
implies storing return value of mlx5_tout_ms() on a ulong
should be fine.

This fixes:

  ERROR: modpost: "__udivdi3" [drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko] undefined!

Fixes: 32def412

 ("net/mlx5: Read timeout values from DTOR")
Link: https://lore.kernel.org/r/20211018172608.1069754-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>