thunderbolt: Rework NFC credits handling

NFC (non flow control) credits is actually 20-bit field so update
tb_port_add_nfc_credits() to handle this properly. This allows us to set
NFC credits for Display Port path in subsequent patches.

Also make sure the function does not update the hardware if the
underlying switch is already unplugged.

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