scripts: checkpatch: Revert dt-binding vendor prefix check regex change

This commit reverts the device tree binding vendor prefix check regular
expression change that was introduced by the following commit:
5b10fac97efdb444b49f5e3c3ac16048ce8661c6

The changed regular expression fails to detect the correctly specified
vendor prefixes in the `vendor-prefixes.txt`.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
1 file changed