sh: Kill off broken PHYSADDR() usage in sh4_flush_dcache_page().

PHYSADDR() runs in to issues in 32-bit mode when we do not have the
legacy P1/P2 areas mapped, as such, we need to use page_to_phys()
directly, which also happens to do the right thing in legacy 29-bit mode.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>