rtc: ds2404: use hw endiannes variable

Converting from hardware to host endiannes was done using reassignment
to the same variable which makes sparse unhappy as it can not verify
the endiannes handling properly. To allow sparse to verify endiannes
handling an explicit __le32 is introduced. Note that this patch does
not change the generated binary (x86_64 and ppc64 binary diff).

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>