ARM: virt: avoid clobbering lr when forcing svc mode

The safe_svcmode_maskall macro is used to ensure that we are running in
svc mode, causing an exception return from hvc mode if required.

This patch removes the unneeded lr clobber from the macro and operates
entirely on the temporary parameter register instead.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[will: updated comment]
Signed-off-by: Will Deacon <will.deacon@arm.com>