rxgen: use unsigned type for max array length

Plain '0' is of type int, i.e., signed, and therefore so is '~0'.
The length of an XDR array is unsigned, so this constant should
be of an unsigned type.

Change-Id: I13f5f94b2f54bc0adcdf2ded1696b797b5205057
Reviewed-on: http://gerrit.openafs.org/11107
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>