Revert "thunderbolt: Prevent crash if non-active NVMem file is read"

This reverts commit 03cd45d2.

Commit 664f0549 ("nvmem: core: use is_bin_visible for permissions")
incidentally adds support for write-only nvmem. Hence, this workaround
is no longer required, so drop it.

Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>