usb: dwc3: core: don't kfree() devm_kzalloc()'ed memory

commit 380f0d28 (usb: dwc3: core: switch event
buffer allocation to devm_kzalloc()) was incomplete
leaving a trailing kfree(evt) in an error exit
path.

Fix this problem by removing the trailing kfree(evt).

Cc: Julia Lawall <julia.lawall@lip6.fr>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>