xilinx_devcfg: Fix "sync word" messages

When (partially) programming the PL, the driver outputs unwanted
messages like "Found normal sync word".

Replace use of "default" printk with pr_debug. This surpresses the
message until explicitly asked for.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>