mfd: ti_am335x_tscadc: Avoid possible deadlock of reg_lock

Since the addition of continuous sampling mode and shared irq support,
the reg_lock lock can be taken with and without interrupts. This patch
uses the *_irq* variant which should be used in order to avaoid a deadlock.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>