[eepro] Avoid unused variable warning in gcc 4.6

From a cursory examination, it appears as though the calculation of
tx_available is redundant, since eepro_transmit() waits for transmit
completion before returning anyway.

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/eepro.c

 		tx_available = tx_start - tx_end;
 	else
 		tx_available = XMT_RAM;
+	assert ( length <= tx_available );
 	last = tx_end;
 	end = last + (((length + 3) >> 1) << 1) + XMT_HEADER;
 	if (end >= (XMT_UPPER_LIMIT << 8)) {