net/mlx5: Use max_num_eqs for calculation of required MSIX vectors

New firmware has defined new HCA capability field called "max_num_eqs",
that is the number of available EQs after subtracting reserved FW EQs.

Before this capability the FW reported the EQ number in "log_max_eqs",
the reported value also contained FW reserved EQs, but the driver might
be failing to load on 320 cpus systems due to the fact that FW
reserved EQs were not available to the driver.

Now the driver has to obtain max_num_eqs value from new FW to get real
number of EQs available.

Signed-off-by: Denis Drozdov <denisd@mellanox.com>
Reviewed-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>