net/mlx5e: Increase VF representors' SQ size to 128

The default size for the VF representors' SQ was too small to handle high
packet rates. Doubling the size from 64 to 128 drastically improves the
packet rate under stress (by about 50%), whereas increasing the size
beyond 128 has not shown to make any further difference.

The impact of the SQ size was measured with UDP traffic, in the following
topology: TG <-> PF <-> TC forwarding <-> VF representor <-> VF in VM
over a single core processing bi-directional traffic, with the following
results:

                                  SQ size of 64:     SQ size of 128:
Packet rate for 64B UDP packets:    860 [Kpps]         1280 [Kpps]
Packet rate for 114B VxLan
encapsulated UDP packets:           320 [Kpps]          500 [Kpps]

Signed-off-by: Gavi Teitz <gavi@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>