ice: store VF relative MSI-X index in q_vector->vf_reg_idx

ice: store VF relative MSI-X index in q_vector->vf_reg_idx

The ice physical function driver needs to configure the association of
queues and interrupts on behalf of its virtual functions. This is done over
virtchnl by the VF sending messages during its initialization phase. These
messages contain a vector_id which the VF wants to associate with a given
queue. This ID is relative to the VF space, where 0 indicates the control
IRQ for non-queue interrupts.

When programming the mapping, the PF driver currently passes this vector_id
directly to the low level functions for programming. This works for SR-IOV,
because the hardware uses the VF-based indexing for interrupts.

This won't work for Scalable IOV, which uses PF-based indexing for
programming its VSIs. To handle this, the driver needs to be able to look
up the proper index to use for programming. For typical IRQs, this would be
the q_vector->reg_idx field.

The q_vector->reg_idx can't be set to a VF relative value, because it is
used when the PF needs to control the interrupt, such as when triggering a
software interrupt on stopping the Tx queue. Thus, introduce a new
q_vector->vf_reg_idx which can store the VF relative index for registers
which expect this.

Use this in ice_cfg_interrupt to look up the VF index from the q_vector.
This allows removing the vector ID parameter of ice_cfg_interrupt. Also
notice that this function returns an int, but then is cast to the virtchnl
error enumeration, virtchnl_status_code. Update the return type to indicate
it does not return an integer error code. We can't use normal error codes
here because the return values are passed across the virtchnl interface.

This will allow the future Scalable IOV VFs to correctly look up the index
needed for programming the VF queues without breaking SR-IOV.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>