net: e1000: use correct helper to do endianness conversion

In struct e1000_rx_desc, field 'length' is declared as
uint16_t, so use le16_to_cpu() to do endianness conversion.

Also drop conversion on 'status' which is declared as
uint8_t.

Signed-off-by: Miao Yan <yanmiaobest@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>