usb: dwc3: Correct clean up code for requests

usb: dwc3: Correct clean up code for requests

For u-boot dwc3 driver the scatter gather list support has been removed
from original linux code. It is correct, since we try to send one request
at a time.
However, the cleanup left spurious break, which caused early exit from
loop at dwc3_cleanup_done_reqs() function. As a result the dwc3_gadget_giveback()
wasn't called and caused USB Mass Storage to hang.

This commit removes this problem and refactor the code to remove superfluous
do { } while(1) loop.

Test HW: Odroid XU3 (with ./test/ums/ums_gadget_test.sh)

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>