gnss: ubx: make struct ubx_gserial_ops static

The structure ubx_gserial_ops is local to the source and does not need
to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'ubx_gserial_ops' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>