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: 0ddcbee2 ("ipq806x: activate ATAG DTB mangle and EA8500 rootblock in dts")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>

target/linux/ipq806x/patches-4.4/996-ATAG_DTB_COMPAT_CMDLINE_MANGLE.patch

@@ -64,9 +64,9 @@ Signed-off-by: Adrian Panella <ianchi74@outlook.com>
 +	ptr = str - 1;
 +
 +	do {
-+		ptr++;
 +		//first find an 'r' at the begining or after a space
 +		do {
++			ptr++;
 +			ptr = strchr(ptr, 'r');
 +			if(!ptr) return dest;
 +