xen/arm: timer: Don't warn if the timer interrupts are high-level

xen/arm: timer: Don't warn if the timer interrupts are high-level

The commit 61d6a84 "xen: arm: Warn if timer interrupts are not level
triggered" added a check to ensure the timer interrupts are level.

Although, the check is only done on low-level interrupts. This will
result to a strange warning on platform using high-level ("WARNING [..]
IRQ is not level triggered").

As high-level interrupt is valid, don't warn on both high-level and
low-level interrupt.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>