rtc: msc313: Fix unintentional sign extension issues with left shift of a u16

Shifting the u16 value returned by readw by 16 bits to the left
will be promoted to a 32 bit signed int and then sign-extended
to an unsigned long. If the top bit of the readw is set then
the shifted value will be sign extended and the top 32 bits of
the result will be set.

Fixes: be7d9c91 ("rtc: Add support for the MSTAR MSC313 RTC")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Romain Perier <romain.perier@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210928134654.991923-1-colin.king@canonical.com