mfd: mc13xxx-core: Initialize the lock prior to using it

After request_threaded_irq() is called there is a chance that an interrupt
may occur before the 'mc13xxx->lock' is initialized, which will trigger a kernel
oops.

In order to prevent that, move the initialization of 'mc13xxx->lock' prior to
requesting the interrupts.

Suggested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>