usb: dwc3: Fix bug in ep disable operation

__dwc3_gadget_ep_disable API doing memset 0 with size
set to DWC3_TRB_NUM. Number of TRBs allocated for gsi
endpoints are less than DWC3_TRB_NUM. This results in
to memory corruption. Fix this bug by introducing
num_trbs member in dwc3_ep structure to save number of
trbs allocated in a dma pool upon dma pool creation.
Ep disable operation will use num_trbs of a dwc3_ep to
perform memset 0.

Change-Id: I94b5865ca22b4e1fde0d2cd8dcb218906327a916
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>