rtc: rx8025: cleanup accessors

Remove useless error messages, at that point, the user already knows
something went wrong but will not be able to do anything about it anyway.
It is also highly unlikely that some registers are readable/writable
but not some other ones.

Also, transform rx8025_read_reg to be more resemblant to
i2c_smbus_read_byte_data()

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>