lapb: Neaten debugging

Enable dynamic debugging and remove a bunch of #ifdef/#endifs.

Add a lapb_dbg(level, fmt, ...) macro and replace the
printk(KERN_DEBUG uses.
Add pr_fmt and remove embedded prefixes.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>