rtc: hym8563: fix a missing check of block data read

When i2c_smbus_read_i2c_block_data() fails, the read data in "buf" could
be incorrect and should not be used. The fix checks if
i2c_smbus_read_i2c_block_data fails, and if so, return its error code
upstream.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>