powerpc/rtas: Fix watchdog driver temperature read functionality

Using the RTAS watchdog driver to read out the temperature crashes
on a PXCAB:

Unable to handle kernel paging request for data at address 0xfe347b50
Faulting instruction address: 0xc00000000001af64
Oops: Kernel access of bad area, sig: 11 [#1]

The wrong usage of "(void *)__pa(&temperature)" in rtas_call() is
removed by using the function rtas_get_sensor() which does the
right thing.

Signed-off-by: Adrian Reber <adrian@lisas.de>
Acked-by: Utz Bacher <utz.bacher@de.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>