ppce500_spin: Replace assert by hw_error (fixes compiler warning)

ppce500_spin: Replace assert by hw_error (fixes compiler warning)

The default case in function spin_read should never be reached,
therefore the old code used assert(0) to abort QEMU.

This does not work when QEMU is compiled with macro NDEBUG defined.
In this case (and also when the compiler does not know that assert
never returns), there is a compiler warning because of the missing
return value.

Using hw_error allows an improved error message and aborts always.

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