rtc: m41t80: replace i2c functions for smbus ones

The driver used i2c_transfer methods to read and set date/time.
The smbus methods should be used.

This commit replaces i2c_transfer functions by i2c_smbus_XX_i2c_block_data
for reading and setting the datetime.

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>