thunderbolt: debugfs: Handle fail reading block

There are cases when reading block of dwords in single transaction fail,
for several reasons, mostly if HW publish to implement all of the dwords,
while actually it doesn't or if some dwords not accessible for read
for security reasons. We handle these cases by trying to read the block,
dword-by-dword, one dword per transaction, till we get a failure.

Signed-off-by: Gil Fine <gil.fine@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>