ipq806x: fix boot hang if cmdline contains words with r in the middle

ipq806x: fix boot hang if cmdline contains words with r in the middle

The ATB DTB mangle code will enter an infinite loop if it encounters a
word in the command line that contains an r in the middle of the word.

Fix this by increasing ptr everytime before invoking strchr, ot avoid
finding the same r again.

This fixes booting at least on Netgear R7500v1, which contains
"ubi.mtd=rootfs" in its commandline, triggering the misbehaviour.

Fixes: 0ddcbee26151 ("ipq806x: activate ATAG DTB mangle and EA8500 rootblock in dts")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>