jtag/mpsse: mpsse_flush should not treat LIBUSB_ERROR_INTERRUPTED as an error

jtag/mpsse: mpsse_flush should not treat LIBUSB_ERROR_INTERRUPTED as an error

LIBUSB_ERROR_INTERRUPTED can happen when (among other things) OpenOCD
process receives a signal like SIGHUP or SIGINT during a call to libusb.
Such situations are expected and should not be treated as an error - the
affected request should just be restarted.

Without this patch applied if a signal arrives during FTDI initialization
procedure we can easily end up (if JTAG speed is low) in situations like
https://review.openocd.org/c/openocd/+/4767. This happens because
fpsse_flush fails due to LIBUSB_ERROR_INTERRUPTED .

It should be noted that the current usage of mpsse_flush should be
revised since it seems that we don't always process error codes returned
by the function.

Change-Id: Ifa063ce828068f8d0371e1c2a864bb6174649848
Signed-off-by: Parshintsev Anatoly <anatoly.parshintsev@syntacore.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7769
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins