ethernet: ucc_geth: add helper to replace repeated switch statements

The translation from the ucc_geth_num_of_threads enum value to the
actual count can be written somewhat more compactly with a small
lookup table, allowing us to replace the four switch statements.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>