uml: fix inconsistence due to tty_operation change

'put_char' of 'struct tty_operations' has changed from 'void' into 'int'.
This can also shut up compiler warnings.

Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: WANG Cong <wangcong@zeuux.org>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>