thunderbolt: Ensure left shift of 512 does not overflow a 32 bit int

The 32 bit int value 512 is being left shifted and then used in a context
that expects the expression to be a larger unsigned long. There may be
a potential integer overflow, so make 512 a UL before shift to avoid
any such issues.

Addresses-Coverity: ("Uninintentional integer overflow")
Fixes: 3b1d8d57 ("thunderbolt: Implement USB3 bandwidth negotiation routines")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>