writeback: Eliminate unintentional inferred latch

writeback: Eliminate unintentional inferred latch

By not assigning to interrupt_out.srr1 in some circumstances, the
writeback_1 process creates an inferred latch, which is not
desirable.  Eliminate it by restructuring the code so
interrupt_out.srr1 is always set, to zeroes if nothing else.

Fixes: bc4d02cb0dcc ("Start removing SPRs from register file", 2022-07-12)
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>