nvme: fix for big endian systems

writel() and co. already include the endian swap;  doing the swap twice
is, er, unhelpful.

Tested on a P4080DS, which boots perfectly fine off NVMe with this.

Signed-off-by: David Lamparter <equinox@diac24.net>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>