IB/mlx5: Optimize u64 division on 32-bit arches

Commit f164be8c ("IB/mlx5: Extend caps stage to handle VAR
capabilities") introduced a straight "/" division of the u64 variable
"bar_size".

This was fixed with commit 685eff51 ("IB/mlx5: Use div64_u64 for
num_var_hw_entries calculation"). However, div64_u64() is redundant here
as mlx5_var_table::stride_size is of type u32.  Make the actual code way
more optimized on 32-bit kernels using div_u64() and fix 80 chars
break-through by the way.

Fixes: 685eff51 ("IB/mlx5: Use div64_u64 for num_var_hw_entries calculation")
Link: https://lore.kernel.org/r/20200217073629.8051-1-alobakin@dlink.ru


Signed-off-by: Alexander Lobakin <alobakin@dlink.ru>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>