devtool: upgrade: drop superfluous call to validate_pn

devtool: upgrade: drop superfluous call to validate_pn

The recipename argument to devtool upgrade specifies an existing recipe,
so by definition the name will be valid (or it won't exist) - we don't
need to validate it ourselves, that's only needed for situations like in
devtool add where we're creating a new recipe.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>