net/mlx4: clean up a type issue

These functions returns type bool, not pointers, so return false instead
of NULL.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/52d0814a-7287-4160-94b5-ac7939ac61c6@moroto.mountain
Signed-off-by: Jakub Kicinski <kuba@kernel.org>