rtc: s35390a: Change buf's type to u8 in s35390a_init

rtc: s35390a: Change buf's type to u8 in s35390a_init

Clang warns:

drivers/rtc/rtc-s35390a.c:124:27: warning: implicit conversion from
'int' to 'char' changes value from 192 to -64 [-Wconstant-conversion]
        buf = S35390A_FLAG_RESET | S35390A_FLAG_24H;
            ~ ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
1 warning generated.

Update buf to be an unsigned 8-bit integer, which matches the buf member
in struct i2c_msg.

https://github.com/ClangBuiltLinux/linux/issues/145
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>