net/mlx5: Use cpumask_available() in mlx5_eq_create_generic()

When CONFIG_CPUMASK_OFFSTACK is unset, cpumask_var_t is not a pointer
but a single element array, meaning its address in a structure cannot be
NULL as long as it is not the first element, which it is not. This
results in a clang warning:

drivers/net/ethernet/mellanox/mlx5/core/eq.c:715:14: warning: address of
array 'param->affinity' will always evaluate to 'true'
[-Wpointer-bool-conversion]
        if (!param->affinity)
            ~~~~~~~~^~~~~~~~
1 warning generated.

The helper cpumask_available was added in commit f7e30f01 ("cpumask:
Add helper cpumask_available()") to handle situations like this so use
it to keep the meaning of the code the same while resolving the warning.

Fixes: e4e3f24b

 ("net/mlx5: Provide cpumask at EQ creation phase")
Link: https://github.com/ClangBuiltLinux/linux/issues/1400
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>