tty: hvc: hvc_vio: Staticify function invoked only by reference

tty: hvc: hvc_vio: Staticify function invoked only by reference

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

 drivers/tty/hvc/hvc_vio.c:181:6: warning: no previous prototype for ‘hvterm_hvsi_hangup’ [-Wmissing-prototypes]
 181 | void hvterm_hvsi_hangup(struct hvc_struct *hp, int data)
 | ^~~~~~~~~~~~~~~~~~

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-33-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>