rtc: m41t80: move m41t80_rtc_mutex to the block where it is used

Without CONFIG_RTC_DRV_M41T80_WDT the compiler complains:

|drivers/rtc/rtc-m41t80.c:76 ‘m41t80_rtc_mutex’ defined but not used [-Wunused-variable]

Move the variable to the block where it is used.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>