ppc/xive2: Reset Generation Flipped bit on END Cache Watch

ppc/xive2: Reset Generation Flipped bit on END Cache Watch

When the END Event Queue wraps the END EQ Generation bit is flipped and the
Generation Flipped bit is set to one.  On a END cache Watch read operation,
the Generation Flipped bit needs to be reset.

While debugging an error modified END not valid error messages to include
the method since all were the same.

Signed-off-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Caleb Schlossin <calebs@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-8-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>