powerpc/windfarm: don't pass const strings to snprintf

Fixes these build warnings:

drivers/macintosh/windfarm_smu_sat.c: In function 'wf_sat_probe':
drivers/macintosh/windfarm_smu_sat.c:290:3: warning: passing argument 1 of 'snprintf' discards qualifiers from pointer target type
include/linux/kernel.h:323:5: note: expected 'char *' but argument is of type 'const char *'
drivers/macintosh/windfarm_smu_sat.c:317:3: warning: passing argument 1 of 'snprintf' discards qualifiers from pointer target type
include/linux/kernel.h:323:5: note: expected 'char *' but argument is of type 'const char *'

Introduced by commit e074d08e ("powerpc/windfarm: const'ify and add
"priv" field to controls & sensors").

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>