powerpc/sysdev: Remove some duplicate prefix in some messages

At the beginning of the file, we have:
   #define pr_fmt(fmt) "xive: " fmt

So, there is no need to duplicate "XIVE:" in debug and error messages.

For the records, these useless prefix have been added in commit
5af50993

 ("KVM: PPC: Book3S HV: Native usage of the XIVE interrupt
controller")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7b8b5915a2c7c1616b33e8433ebe0a0bf07070a2.1665312579.git.christophe.jaillet@wanadoo.fr