Staging: pata_rdc: fix build warnings

struct ata_port_info shouldn't be const, so remove that which fixes up
the compiler warnings.

Cc: Kevin Huang  <Kevin.Huang@rdc.com.tw>
Cc: Tomy Wang <Tomy.Wang@rdc.com.tw>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>