usb: dwc3: trace: show request flags

struct usb_request have 3 flags which might be
important to know about during debug. This patch
shows each of the 3 flags as a single letter:

z -> for zero
s -> short not okay
i -> interrupt

A capital letter means the feature is enabled
while a lower case letter means it is disabled;

Thus 'zsI' indicates that a ZLP is not needed,
that we can accept a short packet and interrupt
for this request should be enabled.

Signed-off-by: Felipe Balbi <balbi@ti.com>