ARM: 7652/1: mm: fix missing use of 'asid' to get asid value from mm->context.id

Fix missing use of the asid macro when getting the ASID from the mm->context.id field.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>