drivers: atmel_maxtouch: Fix IRQ handling in a possible race condition

Possibility of the IRQ being disabled twice if IRQ hander and the
missed interrupt check execute simultaneously. This can case the IRQ
to be disabled forever.

Change-Id: If1d1cac138727c866dcd99b882b778d4b31b1774
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>