ibmvnic: fix kernel build warnings in build_hdr_descs_arr

Fix the following kernel build warnings:
drivers/net/ethernet/ibm/ibmvnic.c:1516: warning: Function parameter or member 'skb' not described in 'build_hdr_descs_arr'
drivers/net/ethernet/ibm/ibmvnic.c:1516: warning: Function parameter or member 'indir_arr' not described in 'build_hdr_descs_arr'
drivers/net/ethernet/ibm/ibmvnic.c:1516: warning: Excess function parameter 'txbuff' description in 'build_hdr_descs_arr'

Signed-off-by: Lijun Pan <lijunp213@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>