rtc: pcf2127: fix pcf2127_nvmem_read/write() returns

These functions should return zero on success.  Non-zero returns are
treated as error.  On some paths, this doesn't matter but in
nvmem_cell_read() a non-zero return would be passed to ERR_PTR() and
lead to an Oops.

Fixes: d6c3029f ("rtc: pcf2127: add support for accessing internal static RAM")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201022070451.GA2817669@mwanda