[pcnet32] Eliminate redundant register read

The value of ( ( x & 0x0c00 ) | 0x0c00 ) is always 0x0c00 regardless
of the value of x, and so the read_csr() is redundant.  (There are no
read side effects for this register, according to the datasheet.)

This line of code originated in Linux kernel 2.3.19pre1 as

  a->write_csr(ioaddr, 80, a->read_csr(ioaddr, 80) | 0x0c00);

and was modified in kernel 2.3.41pre4 to read

  a->write_csr(ioaddr, 80, (a->read_csr(ioaddr, 80) & 0x0C00) | 0x0c00);

In the absence of commit messages, the intention of the code is
unclear.  However, the logic resulting in a fixed value of 0x0c00 has
remained unaltered for over 17 years, and can probably be assumed to
have the correct overall result.

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

src/drivers/net/pcnet32.c

 	if (fset) {
 		a->write_bcr ( ioaddr, 18,
 			( a->read_bcr ( ioaddr, 18 ) | 0x0860 ) );
-		a->write_csr ( ioaddr, 80,
-			( a->read_csr ( ioaddr, 80 ) & 0x0C00) | 0x0C00 );
+		a->write_csr ( ioaddr, 80, 0x0c00 );
 	}
 
 	priv->full_duplex = fdx;