LinuxKPI: pci: rename PCI_IRQ_LEGACY to PCI_IRQ_INTX

PCI_IRQ_LEGACY was added in 366d68f28379 (likely) for rtw88 only.
The define now got renamed and I cannot find any other use but
rtw88 and rtw89.
We keep the old name as comapt while driver updates are in
progress enabling it defining LINUXKPI_VERSION for the two drivers.

Sponsored by; The FreeBSD Foundation
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46822