serial: sifive: Declare PM operations as static

They are only used within this file, so they should have static linkage.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20231113023122.1185407-1-samuel.holland@sifive.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>