imx-vpu: fix checks of IOGetVirtMem() return value

The IOGetVirtMem() routine returns the address of a memory block
or -1 (MAP_FAILED) to indicate an error.

Many callers of this routine tested the return value for <= 0
to detect failure, and at least with a 3G/1G memory split
configured in the kernel, a negative number (!= -1) is a
valid (successful) return value.

Without this patch, the IOSystemInit() will often detect
failure incorrectly.

Change-Id: I8b20ae5f74608307cbb810168024e8863599dbdb
Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Merge remote-tracking branch 'freescale/master' into upstream