NFC: The NFC genl family structure should not be exposed globally

The variable 'nfc_genl_family' is only referenced in this file and
should be marked static to prevent it from being exposed globally.

Quites the sparse warning:

warning: symbol 'nfc_genl_family' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Lauro Ramos Venancio <lauro.venancio@openbossa.org>
Cc: Aloisio Almeida Jr <aloisio.almeida@openbossa.org>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>