net: cdc_ncm: cleanup a type issue in cdc_ncm_setup()

This is harmless but cdc_ncm_setup() returns negative error codes
truncated to u8 values.  There is only one caller and treats all
non-zero returns as errors but doesn't store the the return code.  So
the code works correctly but it's messy and upsets the static checkers.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>