usb: gadget: atmel_usba_udc: Remove redundant platform_set_drvdata()

Commit 0998d063 (device-core: Ensure drvdata = NULL when no
driver is bound) removes the need to set driver data field to
NULL.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>