cassini: Make missing firmware non-fatal

The firmware patch for the Saturn PHY fixes a bug, but is not absolutely
essential.  And its licence is unclear, so it is not included in all
distributions.  Just log an error message and continue if it is missing
or invalid.

References: http://bugs.debian.org/712674


Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Jose Andres Arias Velichko <Andres.Arias@PaisLinux.net> (against 3.2)
Signed-off-by: David S. Miller <davem@davemloft.net>