rtc: cmos: Fix return value of nvmem callbacks

rtc: cmos: 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.

cmos_nvram_read()/cmos_nvram_write() currently return the number of
bytes read or written, fix to return 0 on success and -EIO incase number
of bytes requested was not read or written.

Fixes: 8b5b7958fd1c ("rtc: cmos: use generic nvmem")
Cc: stable@vger.kernel.org
Signed-off-by: Joy Chakraborty <joychakr@google.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20240612083635.1253039-1-joychakr@google.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>