powerpc/fadump: make symbol 'rtas_fadump_set_regval' static

Fix sparse warnings:

arch/powerpc/platforms/pseries/rtas-fadump.c:250:6: warning:
 symbol 'rtas_fadump_set_regval' was not declared. Should it be static?

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210408062012.85973-1-pulehui@huawei.com