[epic100] Avoid unused variable warning in gcc 4.6

Reported-by: Ralph Giles <giles@thaumas.net>
Tested-by: Ralph Giles <giles@thaumas.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/epic100.c

 epic100_open(void)
 {
     int mii_reg5;
-    int full_duplex = 0;
     unsigned long tmp;
 
     epic100_init_ring();
 
     mii_reg5 = mii_read(phys[0], 5);
     if (mii_reg5 != 0xffff && (mii_reg5 & 0x0100)) {
-	full_duplex = 1;
 	printf(" full-duplex mode");
 	tmp |= TC_LM_FULL_DPX;
     } else