ax88796: export ax_NS8390_init() hook

The block I/O code for the new X-Surf 100 ax88796 driver needs
ax_NS8390_init() for error fixup in its block_output function.

Export this static function through the ax_NS8390_reinit()
wrapper so we can lose the lib8380.c include in the X-Surf 100
driver.

[arnd: add the declaration in the header to avoid a
 -Wmissing-prototypes warning]
Fixes: 861928f4

 ("net-next: New ax88796 platform
driver for Amiga X-Surf 100 Zorro board (m68k)")
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>