ARM: 9354/1: ptrace: Use bitfield helpers

The isa_mode() macro extracts two fields, and recombines them into a
single value.

Make this more obvious by using the FIELD_GET() helper, and shifting the
result into its final resting place.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>