rtc: isl1208: Fix return value of nvmem callbacks

rtc: isl1208: Fix return value of nvmem callbacks

Read/write callbacks registered with nvmem core expect 0 to be returned
on success and a negative value to be returned on failure.

isl1208_nvmem_read()/isl1208_nvmem_write() currently return the number of
bytes read/written on success, fix to return 0 on success and negative on
failure.

Fixes: c3544f6f51ed ("rtc: isl1208: Add new style nvmem support to driver")
Cc: stable@vger.kernel.org
Signed-off-by: Joy Chakraborty <joychakr@google.com>
Link: https://lore.kernel.org/r/20240612080831.1227131-1-joychakr@google.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>