dm: avoid dev->req_seq overflow

Since dev->req_seq value is initialized from "reg" property of fdt node,
there is posibility, that address value contained in fdt is greater than
INT_MAX, and then value in dev->req_seq is negative which led to probe()
fail.

This patch fix this problem by ensuring that req_seq is positive, unless
it's one of errno codes.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>