net: microchip: vcap: Add vcap_mod_rule

Add the function vcap_mod_rule which allows to update an existing rule
in the vcap. It is required for the rule to exist in the vcap to be able
to modify it.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>