Staging: tty: fix build with epca.c driver

I forgot to move the digi*.h files from drivers/char/ to
drivers/staging/tty/

Thanks to Randy for pointing out the issue.
Reported-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>