[ARM] 2974/1: fix ARM710 swi bug workaround

Patch from Nicolas Pitre

Either no one is using an ARM710 with recent kernels, or all ARM710s
still in use are not afflicted by this swi bug.  Nevertheless, the code
to work around the ARM710 swi bug is itself currently buggy since it
uses r8 as a pointer to S_PC while in fact it holds the spsr content
these days. Fix that, and simplify the code as well.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>