[realtek] Use read-modify-write to check for C+ Command register

Some bits in the C+ Command register are always one.  Testing for the
presence of the register must allow for this.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/realtek.c

 	 * Try to enable C+ mode and PCI Dual Address Cycle (for
 	 * 64-bit systems), if supported.
 	 */
-	cpcr = ( RTL_CPCR_DAC | RTL_CPCR_MULRW | RTL_CPCR_CPRX |
-		 RTL_CPCR_CPTX );
+	cpcr = readw ( rtl->regs + RTL_CPCR );
+	cpcr |= ( RTL_CPCR_DAC | RTL_CPCR_MULRW | RTL_CPCR_CPRX |
+		  RTL_CPCR_CPTX );
 	writew ( cpcr, rtl->regs + RTL_CPCR );
 	check_cpcr = readw ( rtl->regs + RTL_CPCR );
 
 		rtl->have_phy_regs = 1;
 		rtl->tppoll = RTL_TPPOLL_8169;
 	} else {
-		if ( check_cpcr == cpcr ) {
+		if ( ( check_cpcr == cpcr ) && ( cpcr != 0xffff ) ) {
 			DBGC ( rtl, "REALTEK %p appears to be an RTL8139C+\n",
 			       rtl );
 			rtl->tppoll = RTL_TPPOLL_8139CP;