thunderbolt: Fix error code in tb_port_is_width_supported()

This function is type bool, and it's supposed to return true on success.
Unfortunately, this path takes negative error codes and casts them to
bool (true) so it's treated as success instead of failure.

Fixes: 91c0c120 ("thunderbolt: Add support for lane bonding")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>