net: cdc_ncm: fix typo in test for supported formats

There is a typo here where we test for USB_CDC_NCM_NTH32_SIGN instead
of USB_CDC_NCM_NTB32_SUPPORTED.  The test probably still works as
written because 0x686D636E has (1 << 1) set and doesn't have (1 << 0)
set.

Fixes: f8afb73d

 ('net: cdc_ncm: factor out one-time device initialization')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>