net: microchip: vcap: Add vcap_get_rule

Add function vcap_get_rule which returns a rule based on the internal
rule id.
The entire functionality of reading and decoding the rule from the VCAP
was inside vcap_api_debugfs file. So move the entire implementation in
vcap_api as this is used also by vcap_get_rule.

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