sfxge: IPv4 Tx checksum offload may be disabled in fact

Split IFCAP_HWCSUM to IFCAP_RXCSUM and IFCAP_TXCSUM to highlight Tx and Rx.

Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D2541