Fix missing `PKG_SOURCE` variable leading to a
`Download/default is missing the FILE field` error in the buildroot.
Also remove now uneeded variable as the default one matches the git clone
structure. Finally use the common variable assignment style for
`PKG_SOURCE_PROTO` and `PKG_SOURCE_VERSION`.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>