tty: hvc: hvc_opal: Staticify function invoked by reference

tty: hvc: hvc_opal: Staticify function invoked by reference

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

 drivers/tty/hvc/hvc_opal.c:106:6: warning: no previous prototype for ‘hvc_opal_hvsi_hangup’ [-Wmissing-prototypes]

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
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/20201104193549.4026187-34-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>