tty: fix a build failure on sparc

On sparc, there is a build failure:

drivers/tty/serial/8250/8250.c:48:21: error: suncore.h: No such file or directory
drivers/tty/serial/8250/8250.c:3275: error: implicit declaration of function 'sunserial_register_minors'
drivers/tty/serial/8250/8250.c:3305: error: implicit declaration of function 'sunserial_unregister_minors'

this is due to commit 9bef3d41
(serial: group all the 8250 related code together) moved these files
into 8250/ subdirectory, but forgot to change the reference
to drivers/tty/serial/suncore.h.

Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>