For targets with NO_XIP ltq_mtd->map[i].phys equals -1 and devm_ioremap fails.
Fix this by using pdev->resource[i].start instead.

Signed-off-by: Matti Laakso <malaakso@elisanet.fi>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44450 3c298f89-4303-0410-b956-a3cf2f4a3e73