suppress verbose debug messages: change printk() to DEBUG_MSG()

[jc: no signoff, added my own]
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Signed-off-by: Jonathan Corbet <corbet@lwn.net>