NFC: NFC core layer should not set the target_idx

The NFC core layer should not set the target_idx.
Instead, the driver layer (e.g. NCI, PN533) should set the
target_idx, so that it will be able to identify the target
when its I/F (e.g. activate_target) is called.
This is required in order to support multiple targets.
Note that currently supported drivers (PN533 and NCI) don't
use the target_idx in their implementation.

Signed-off-by: Ilan Elias <ilane@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>