ARM: KVM: Cleanup exception injection

David Binderman reported that the exception injection code had a
couple of unused variables lingering around.

Upon examination, it looked like this code could do with an
anticipated spring cleaning, which amounts to deduplicating
the CPSR/SPSR update, and making it look a bit more like
the architecture spec.

The spurious variables are removed in the process.

Reported-by: David Binderman <dcb314@hotmail.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>