drivers/ps3: Fix ps3-vuart null dereference

On the unlikely event that drv is null, the current code will
perform a null pointer dereference with it when printing a dev_dbg
message.  Instead, the BUG_ON check on drv should be performed
before we emit the dev_dbg message.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>