arm64: io: Fix memset_io optimization

memset_io optimization for 8bytes write missed to use the
internal variable formed for writing whole 8bytes with
same pattern of the bytes and instead it was only writing
the one byte with the memset byte pattern.

Use the internal variable 'qc' instead while doing
the quad writes.

Change-Id: I7129fd91f2da3cfa717e9f4827dca84c330bac74
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>