lp8727_charger: Change i2c functions

On writing single byte via i2c, use i2c_smbus_write_byte_data()
rather than i2c_smbus_write_i2c_block_data().

Name changes :
lp8727_i2c_read() -> lp8727_read_bytes()
lp8727_i2c_write() -> removed
lp8727_i2c_read_byte() -> lp8727_read_byte()
lp8727_i2c_write_byte() -> lp8727_write_byte()

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>