net/mlx5: Use read-modify-write when changing PCMR register values

Currently changing a PCMR field is done by setting the field in a
zeroed buffer, zeroing other unrelated fields.
Fix this behaviour by modifying only the required field after first
reading the current register values, as a pre-step towards using more
fields in PCMR register.

Signed-off-by: Eli Britstein <elibr@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>