usb: dwc3: Avoid NULL pointer access

dwc3_calc_trbs_left function is calling dwc3_ep_prev_trb
which can return NULL if trb pool is empty. This change adds
check for the return value of dwc3_ep_prev_trb function to
avoid NULL pointer dereference.

Change-Id: Id11d5ffd23da7c070d5e0b26842d8738b073b212
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>