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 <[e-mail]>
Cc: Jiri Slaby <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_buffer.c

···
 }
 
 /**
- *	tty_buffer_request_room		-	grow tty buffer if needed
+ *	__tty_buffer_request_room		-	grow tty buffer if needed
  *	@port: tty port
  *	@size: size desired
  *	@flags: buffer flags if new buffer allocated (default = 0)