acpi_os_vprintf: Use printk_get_level() to avoid unnecessary KERN_CONT

acpi_os_vprintf currently always uses a KERN_CONT prefix which may be
followed immediately by a proper KERN_<LEVEL>.  Check if the buffer
already has a KERN_<LEVEL> at the start of the buffer and avoid the
unnecessary KERN_CONT.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>