serial: use SGI_HAS_ZILOG for IP22_ZILOG depends

- Use SGI_HAS_ZILOG for IP22_ZILOG depends
- remove IP22 from description, because the driver works on more than
  IP22 SGI machines

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>