Better lens info for generic canon lenses.

As was pointed out by Robert Krawitz, using Exif.CanonCs.LensType gives
a generic value "65535" for many lenses:

PowerShot S3:
Exif.CanonCs.LensType  Short  1  (65535)
Exif.CanonCs.Lens      Short  3  6.0 - 72.0 mm

PowerShot SX1:
Exif.CanonCs.LensType  Short  1  (65535)
Exif.CanonCs.Lens      Short  3  5.0 - 100.0 mm

PowerShot A75:
Exif.CanonCs.LensType  Short  1  (65535)
Exif.CanonCs.Lens      Short  3  5.4 - 16.2 mm

EOS 300D:
Exif.CanonCs.LensType  Short  1  (65535)
Exif.CanonCs.Lens      Short  3  28.0 - 135.0 mm

EOS 20D:
Exif.CanonCs.LensType  Short  1  (65535)
Exif.CanonCs.Lens      Short  3  12.0 - 24.0 mm

In these cases, we now use "Canon generic" + Exif.CanonCs.Lens.