tty: tty_buffer: Fix incorrectly documented function __tty_buffer_request_room()

tty: tty_buffer: Fix incorrectly documented function __tty_buffer_request_room()

Fixes the following W=1 kernel build warning(s):

 drivers/tty/tty_buffer.c:259: warning: expecting prototype for tty_buffer_request_room(). Prototype was for __tty_buffer_request_room() instead

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210520121906.3468725-6-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>