staging: greybus: svc_watchdog: replace printk() with pr_err()

Fix coding style issue caught by checkpatch.pl related to the following
warning:
	- "WARNING: Prefer [subsystem eg: netdev]_err([subsystem]dev, ... then
dev_err(dev, ... then pr_err(...  to printk(KERN_ERR ."

Signed-off-by: Emmanuil Chatzipetru <chatzi.emanuel@gmail.com>
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>