NFC: digital: digital_tg_send_sensf_res() can be static

Fixes sparse hint:

net/nfc/digital_technology.c:640:5: sparse: symbol 'digital_tg_send_sensf_res'
was not declared. Should it be static?

Cc: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>