rtc: m41t80: remove excess kerneldoc

Fix the following warning:
drivers/rtc/rtc-m41t80.c:716: warning: Excess function parameter 'inode' description in 'wdt_ioctl'

Link: https://lore.kernel.org/r/20191122102212.400158-6-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>