[ARM] 3381/1: ixp2000: fix slowport write timing control register fields

Patch from Lennert Buytenhek

The original version of the chip docs had the PW and SU fields in
the slowport write timing control register accidentally reversed.
This is mentioned in the errata (documentation change #4) and fixed
in newer docs.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>