spi: lp-8841: return correct error code from probe

The spi_lp8841_rtc_probe() function misses an initialization of the
return code when it fails to get its memory resource, as gcc notices:

drivers/spi/spi-lp8841-rtc.c: In function 'spi_lp8841_rtc_probe':
drivers/spi/spi-lp8841-rtc.c:239:9: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This changes the code to propagate the error from devm_ioremap_resource().

Fixes: 7ecbfff6 ("spi: master driver to enable RTC on ICPDAS LP-8841")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Sergei Ianovich <ynvich@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>