clocksource: vt8500: Remove deprecated IRQF_DISABLED

This patch removes the use of the IRQF_DISABLED flag.

It's a NOOP since 2.6.35 and it will be removed one day.

[dlezcano] : slightly changed the changelog

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>