ice: avoid rebuilding if MSI-X vector count is unchanged

Commit 05c16687 ("ice: set MSI-X vector count on VF") added support to
change the vector count for VFs as part of ice_sriov_set_msix_vec_count().
This function modifies and rebuilds the target VF with the requested number
of MSI-X vectors.

Future support for live migration will add a call to
ice_sriov_set_msix_vec_count() to ensure that a migrated VF has the proper
MSI-X vector count. In most cases, this request will be to set the MSI-X
vector count to its current value. In that case, no work is necessary.
Rather than requiring the caller to check this, update the function to
check and exit early if the vector count is already at the requested value.
This avoids an unnecessary VF rebuild.

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