extcon: rt8973: Add IRQF_ONESHOT to interrupt flags

This patch adds IRQF_ONESHOT if no primary handler is provided for request
threaded irq.

Signed-off-by: Saurabh Sengar <saurabh.truth@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>