Update class-wc-meta-box-product-data.php

I ran into trouble updating a product SKU where I was simply changing one character from lowercase to uppercase.

WC was returning the "Product SKU must be unique" error because the SQL to check uniqueness wasn't discounting the current post ID from its check. See additional SQL I have inserted at line 1009. Also added LIMIT 1 to speed up query.

Cheers, Phil :)