NFC: digital: Set current target active on activate_target() call

The curr_protocol field of nfc_digital_dev structure used to determine
if a target is currently active was set too soon, immediately when a
target is found. This is not good since there is no other way than
deactivate_target() to reset curr_protocol and if activate_target() is
not called, the target remains active and it's not possible to put the
device in poll mode anymore.

With this patch curr_protocol is set when nfc core activates a target,
puts a device up, or when an ATR_REQ is received in target mode.

Signed-off-by: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>