bhyve/pci_e82545.c: squelch gcc warning for noreturn procedure

Gcc complained that e82545_tx_thread has a return type declared but
doesn't return anything. Annotate the procedure with _Noreturn.

Reviewed by: grehan
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11774