rx: Fix test for end of call queue for LWP

rx: Fix test for end of call queue for LWP

Commit 6ad3d646 (rx: Correctly test for end of call queue) fixed a
broken end-of-queue check in rx_GetCall, but it only fixed the
RX_ENABLE_LOCKS version of rx_GetCall. The non-locks version (i.e. the
LWP version) still had this bug. Fix it for the LWP case, to avoid
some rare cases where an Rx call can get stuck in the incoming queue.

Also remove the comment added by commit 170dbb3c (rx: Use opr queues),
since we're fixing the mentioned problem.

Change-Id: I5b96d97d9aba7bc4b383133b2136f949f3ed22bc
Reviewed-on: https://gerrit.openafs.org/13880
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>