rust: init: Fix generics in *_init! macros

The match pattern for a optional trailing comma in the list of generics
is erroneously repeated in the code block resulting in following error:

| error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
|    --> rust/kernel/init.rs:301:73
|     |
| 301 |         ::pin_init::try_pin_init!($(&$this in)? $t $(::<$($generics),* $(,)?>)? {
|     |                                                                         ^^^

Remove "$(,)?" from all code blocks in the try_init! and try_pin_init!
definitions.

Cc: stable@vger.kernel.org
Fixes: 578eb8b6 ("rust: pin-init: move the default error behavior of `try_[pin_]init`")
Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250628-rust_init_trailing_comma-v1-1-2d162ae1a757@jannau.net


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>