spapr: Use named enum for function remove_hpte

The function returned a target_ulong which was made from unnamed enum
values. The target_ulong was then assigned to an int variable which
was used in a switch statement.

Using a named enum in both cases makes reviews easier.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Alexander Graf <agraf@suse.de>