USB: ci13xxx_udc: Retire pending requests while flushing an endpoint

Rmnet and dun function drivers are designed to start fresh upon MODEM
restart.  The current code is flushing endpoints without freeing
requests.  New requests are allocated again after MODEM is powered up.
All accumulated requests are freed only upon cable disconnect.  If
MODEM is restarted multiple times without disconnecting the cable, we
are running low memory issues.

A pending request can not be freed without dequeuing it.  Dequeuing
all requests in a loop is a heavy operation and involves changing
multiple function drivers.  All pending requests are retired upon
disabling an endpoint.  But an endpoint can not be disabled in
configured state.  Hence retire pending requests while flushing an
endpoint.

CRs-Fixed: 489047
Change-Id: I3440a204ad8c4beb043b1068cb1d3965be96e9b7
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Sridhar Gujje <sgujje@codeaurora.org>