thunderbolt: Make tb_switch_alloc() return ERR_PTR()

In order to detect possible connections to other domains we need to be
able to find out why tb_switch_alloc() fails so make it return ERR_PTR()
instead. This allows the caller to differentiate between errors such as
-ENOMEM which comes from the kernel and for instance -EIO which comes
from the hardware when trying to access the possible switch.

Convert all the current call sites to handle this properly.

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