[POWERPC] Use strcasecmp() rather than strncasecmp() when determining device node compatibility

The current code assumes "foo-bar" must always be compatible with a node
compatible with "foo", which breaks device trees where this is not so.

The "case" part is also wrong according to Open Firmware, but it's more
likely to have drivers and/or device trees depending on it, and thus
needs to be handled more carefully.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>