net: chelsio: remove unused function calc_tx_descs

The inlined helper function calc_tx_descs is not used and is redundant.
Remove it.

Cleans up clang scan build warning:
drivers/net/ethernet/chelsio/cxgb4/sge.c:814:28: warning: unused
function 'calc_tx_descs' [-Wunused-function]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>