rtc: mt6397: Remove unused member dev

Removing the struct member "dev" in mt6397 RTC driver because it's not
initialized and the only usage is for one debugging message.

Also fixed a typo in the error message.

Signed-off-by: Fei Shao <fshao@chromium.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201008093414.1911699-1-fshao@chromium.org