thunderbolt: Remove redundant assignment to variable len

The variable len is assigned a value that is never read. It is re-assigned
a new value in the following do-while loop and never referenced after
the loop. The assignment is redundant and can be removed.

Cleans up clang scan build warning:
drivers/thunderbolt/xdomain.c:344:2: warning: Value stored to 'len' is
never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>