usb: ci_udc: fix emissions of ZLPs

Commit 6a132416 "ci_udc: Update the ci_udc driver to support bulk
transfers" caused the value of "len" to change without updating subsquent
users of that variable in ci_ep_submit_next_request(). This caused the
code that detects when to emit ZLPs (Zero Length Packets) never to
trigger, which in turn caused host timeouts when a ZLP was required,
which in turn broke tests/dfu/, even despite the assertion in that
commit's description that "These changes are tested for both the DFU and
lthor."

Fix this by modifying the added dtd iteration code not to modify "len",
but rather to keep state in a separate variable. Rename the variables
while we're at it so they describe their purpose better.

Fixes: 6a132416 ("ci_udc: Update the ci_udc driver to support bulk transfers")
Cc: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>