rtc: isl1208: Fix unintended clear of SR bits

After successful
sr = isl1208_i2c_set_regs(client, 0, regs, ISL1208_RTC_SECTION_LEN);
sr will be 0.
As a result
sr = i2c_smbus_write_byte_data(client, ISL1208_REG_SR,
			sr & ~ISL1208_REG_SR_WRTC);
is equal to
sr = i2c_smbus_write_byte_data(client, ISL1208_REG_SR, 0);
which clears all flags in SR.

Add an additional read of SR, to have value of SR in sr again.

Signed-off-by: Denis Osterland <Denis.Osterland@diehl.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>