ice: rename ice_sriov.c to ice_vf_mbx.c

ice: rename ice_sriov.c to ice_vf_mbx.c

The ice_sriov.c file primarily contains code which handles the logic for
mailbox overflow detection and some other utility functions related to
the virtualization mailbox.

The bulk of the SR-IOV implementation is actually found in
ice_virtchnl_pf.c, and this file isn't strictly SR-IOV specific.

In the future, the ice driver will support an additional virtualization
scheme known as Scalable IOV, and the code in this file will be used
for this alternative implementation.

Rename this file (and its associated header) to ice_vf_mbx.c, so that we
can later re-use the ice_sriov.c file as the SR-IOV specific file.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>