usb: gadget: diag: Use controller's driver ZLP support on IN endpoint

Currently usb diag function driver is explicitly queueing ZLP request
when size of buffer (i.e. req->legth) is equal to endpoint's maxpacket
size (i.e. ep->maxpacket). USB controller driver does support ZLP
functionality if req->zero is set to true. Hence use this functionality
and remove explicitly queueing ZLP request from USB diag driver.

Change-Id: I224474c6e07b9153da56fafc04d5d04abb8e246b
Signed-off-by: Mayank Rana <mrana@codeaurora.org>