Fix uart_set_ldisc() function type

Commit 64e9159f

 ("serial_core:
uart_set_ldisc infrastructure") introduced the ability for low-level
serial drivers to be informed when the tty ldisc changes.

However, the actual tty-layer function that does this callback for
serial devices was declared with the wrong type, having a spurious and
unused 'ldisc' argument.

This fixed the resulting compiler warning by just removing it.

Acked-by: Blithering Idiot <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>