tty: make tty_set_operations an inline

Since commit f34d7a5b (tty: The big operations rework) in 2008,
tty_set_operations() is a simple one-line assignment. There is no reason
for this to be an exported function, hence move it to a header and make
an inline from that.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210723074317.32690-7-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>