sw64: lib: fix __iowrite64_copy()

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56OLG



--------------------------------

The mb() is supposed to be in the loop as what __iowrite32_copy()
does. Besides, CONFIG_64BIT is always true for SW64, so remove the
redundant #if statement.

Signed-off-by: Dai Xin <daixin@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>