rx: Fix wrong loop iterator in rx_clearProcessRPCStats

Fix the use of an incorrect variable as loop iterator which would have
resulted in a crash.

Caught by clang analyzer.

Change-Id: I785d06b9be0915bc540c37b43d1001a562a7cde4
Reviewed-on: http://gerrit.openafs.org/9202
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>