dwc3: gadget: Add NULL check against endpoint's desc in set_halt()

In few instances, dwc3_gadget_ep_set_halt() API is called after USB
endpoint is disabled which results into device crash with NULL pointer
dereference. USB endpoint's descriptors are set to NULL as part of USB
endpoint disable functionality. Hence fix this issue by adding NULL
pointer check against endpoint's descriptor.

CRs-Fixed: 829034
Change-Id: I2e1f67571ed202fab21655bd8f13fc54706419a7
Signed-off-by: Mayank Rana <mrana@codeaurora.org>