KVM: arm64: selftests: Fix xVAL init in arch_timer_edge_cases

KVM: arm64: selftests: Fix xVAL init in arch_timer_edge_cases

arch_timer_edge_cases hits the following assertion in < 10% of the test runs:

==== Test Assertion Failure ====
  arm64/arch_timer_edge_cases.c:490: timer_get_cntct(timer) >= DEF_CNT + (timer_get_cntfrq() * (uint64_t)(delta_2_ms) / 1000)
  pid=17110 tid=17110 errno=4 - Interrupted system call
     1  0x0000000000404ec7: test_run at arch_timer_edge_cases.c:945
     2  0x0000000000401fa3: main at arch_timer_edge_cases.c:1074
     3  0x0000ffffa774b587: ?? ??:0
     4  0x0000ffffa774b65f: ?? ??:0
     5  0x000000000040206f: _start at ??:?
  timer_get_cntct(timer) >= DEF_CNT + msec_to_cycles(delta_2_ms)

Enabling the timer without proper xval initialization in set_tval_irq()
resulted in an early interrupt during timer reprogramming. Make sure
to set the xval before setting the enable bit.

Signed-off-by: Sebastian Ott <sebott@redhat.com>
Link: https://lore.kernel.org/r/20250605103613.14544-4-sebott@redhat.com
Signed-off-by: Marc Zyngier <maz@kernel.org>