rtc: isl1208: Use i2c block read/write routines

The Linux i2c layer has functions to execute common SMBUS/I2C
transactions.  The register access code here is identical to the I2C
read/write block data routines.

Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>