dwc: improve IPv4 transmit checksum offloading

This patch provides two improvements for TCP/IPv4 and UDP/IPv4
transmit checksum offloading:
(1) Use *CIC_SEG instead of *CIC_FULL, since FreeBSD always provides

a pseudo header checksum.

(2) Don't make transmit IPv4 header checksum offloading a prerequisite

for TCP/IPv4 or UDP/IPv4 transmit checksum offloading.

This is the root cause of PR 291696, since right now the epair
interface does not support transmit IPv4 header checksum offloading,
but TCP/IPv4 and UDP/IPv4 transmit checksum offloading.

PR: 291696
Reviewed by: Timo Voelker
Tested by: Marek Benc
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54395