parisc: Do not hardcode registers in checksum functions

[ Upstream commit 52b2d91752a82d9350981eb3b3ffc4b325c84ba9 ]

Do not hardcode processor registers r19 to r22 as scratch registers.
Instead let the compiler decide, which may give better optimization
results when the functions get inlined.

Signed-off-by: Helge Deller <deller@gmx.de>
Stable-dep-of: a2abae8f0b63 ("parisc: Fix ip_fast_csum")
Signed-off-by: Sasha Levin <sashal@kernel.org>