ata: pata_ns87415: mark ns87560_tf_read static

The global function triggers a warning because of the missing prototype

drivers/ata/pata_ns87415.c:263:6: warning: no previous prototype for 'ns87560_tf_read' [-Wmissing-prototypes]
  263 | void ns87560_tf_read(struct ata_port *ap, struct ata_taskfile *tf)

There are no other references to this, so just make it static.

Fixes: c4b5b7b6 ("pata_ns87415: Initial cut at 87415/87560 IDE support")
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>