ARM: KVM: Change kvm_call_hyp return type to unsigned long

Having u64 as the kvm_call_hyp return type is problematic, as
it forces all kind of tricks for the return values from HYP
to be promoted to 64bit (LE has the LSB in r0, and BE has them
in r1).

Since the only user of the return value is perfectly happy with
a 32bit value, let's make kvm_call_hyp return an unsigned long,
which is 32bit on ARM.

This solves yet another headache.

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