MAINTAINERS/vsprintf: Update link to printk git tree

printk git tree has moved to printk/linux.git in February 2020.

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220105094157.26216-2-pmladek@suse.com