sparc64: vcc: Check for IS_ERR() instead of NULL

The tty_alloc_driver() function never returns NULL, it returns error
pointers on error.

Fixes: ce808b74 ("sparc64: vcc: TTY driver initialization and cleanup")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>