S2io: Added support set_mac_address driver entry point

- Added set_mac_address driver entry point
- Copying permanent mac address to dev->perm_addr
- Incorporated following review comments from Jeff
     - Converted the macro to a function and removed call to memset
     - regarding function naming convention, for all callbacks and entry points
       will have 's2io_' prefix and helper functions will have 'do_s2io_' prefix.

Signed-off-by: Sreenivasa Honnur <sreenivasa.honnur@neterion.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>