usb: gadget: uvc: refactor the check for a valid buffer in the pump worker

By toggling the condition check for a valid buffer, the else path
can be completely avoided.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20240214-uvc-gadget-cleanup-v1-2-de6d78780459@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>