net/mlx5: E-Switch, Consolidate eswitch function number of VFs

Enabled number of VFs is key for eswich manager to do flow steering
initialization and vport configurations. However, the number of
enabled VFs may come from two sources as below.

PF: num of VFs is provided by enabled SR-IOV of itself.
ECPF: num of VFs is provided by enabled SR-IOV from its peer PF. And
      SR-IOV can't be enabled from ECPF itself.

Current driver handles the two cases in different stages and passing
the number of enabled VFs among a large scope of internal functions.
It is usually hard to find out where is the real number of VFs from
due to layers of argument pass-in.

This patch consolidated that number from the entry point of doing
eswitch setup, and maintained a copy so that eswitch functions can
refer to it directly.

Eswitch driver shall always use this number when referring to enabled
number of VFs, don't use other numbers such as from SR-IOV.

Signed-off-by: Bodong Wang <bodong@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>