rtc: ds1307: fix ds1307_native_smbus_read_block_data function

The i2c_smbus_read_i2c_block_data function returns 0 on success, not the
number of bytes written.

Hence, when there are 32 bytes or less to send, the
ds1307_native_smbus_write_block_data function returns 0 on success,
while it returns the number of bytes when there are more than 32.

The ds1307_write_block_data always returns the number of bytes on
success.

Signed-off-by: Nicolas Boullis <nboullis@debian.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>