QStringRef: fix some QStringRef <> const char * relational operators

Their implementations were swapped.

Found while extending tst_qstringapisymmetry, which only exists in
dev, so test will be added to dev once this change has merged up.

[ChangeLog][QtCore][QStringRef] Fixed relational operators against
(const char*) to return the correct result.

Change-Id: I3f331037571b9a543a6885802836b768143d1c1a
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>