rtc: rv3028: factorize EERD bit handling

Both rv3028_eeprom_write and rv3028_eeprom_read enable EERD before sending
commands to the EEPROM and restore it afterwards. Factorize this code.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201009153101.721149-3-alexandre.belloni@bootlin.com