[thunderx] Don't disable NIC when exiting from iPXE

According to ThunderX Errata G-17560, NIC_PF_CFG[ENA] bit should not
be cleared at exit.  This allows other drivers to access the NIC regs
correctly.

Signed-off-by: Konrad Adamczyk <konrad.adamczyk@cavium.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/thunderx.c

 	/* Remove from list of physical functions */
 	list_del ( &pf->list );
 
-	/* Disable physical function */
-	writeq ( 0, ( pf->regs + TXNIC_PF_CFG ) );
-
 	/* Unmap registers */
 	iounmap ( pf->regs );