M41T80_REG_WDAY
tm->tm_wday = buf[M41T80_REG_WDAY] & 0x07;
buf[M41T80_REG_WDAY] = tm.tm_wday;
val = i2c_smbus_read_byte_data(client, M41T80_REG_WDAY);
buf[M41T80_REG_WDAY] |= (val & 0xf0);
M41T80_REG_WDAY : M41T80_REG_SQW;
M41T80_REG_WDAY : M41T80_REG_SQW;