of/flattree: fix of_flat_dt_is_compatible() to match the full compatible string

With the current string comparison, a device tree compatible of "foo-bar"
would match as compatible with a driver looking for "foo".  This patch
fixes the function to use the of_compat_cmp() macro so that it does the
right thing on all platforms (If sparc ever uses this code, it will still
want the strncasecmp() behaviour).

Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>