usb: dwc2: gadget: rename sent_zlp to send_zlp

This flag is set before sending the zlp. So use present tense instead
of the past tense.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>