powerpc/wdrtas: Update wdrtas_get_interval to use rtas_data_buf

powerpc/wdrtas: Update wdrtas_get_interval to use rtas_data_buf

The buffer passed to the ibm,get-system-parameter RTAS call must be
in the RMA.  To ensure we pass an address in the RMA use rtas_data_buf
for the actual RTAS call and then copy the result to value.  We can't
just make it static because this can be compiled in as a module.

Also add the WDRTAS_SP_SPI_LEN so we don't litter '4' throughout the
function.

Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Tested-by: Adrian Reber <adrian@lisas.de>
Acked-by: Utz Bacher <utz.bacher@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>