NFC: netlink: Use error code from nfc_activate_target()

It looks like a typo to assign a return code to a variable which is not
used. Found due to a compiler warning:

net/nfc/netlink.c: In function ‘nfc_genl_activate_target’:
net/nfc/netlink.c:903:6: warning: variable ‘rc’ set but not used [-Wunused-but-set-variable]
  int rc;
        ^~

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>