Bluetooth: properly use pr_fmt() on lib.c

Bluetooth: properly use pr_fmt() on lib.c

pr_fmt() is either defined or we redefine it. Typically
drivers define it prior to including printk.h but this
is done under the assumption that no other subsystem
it uses has already defined pr_fmt(). In such cases
pr_fmt() should be undefined and redefined.

Doing this properly shaves down compilation time quite
considerably.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>