Add byte_enable for ocp_read_word() to replace reading 4
bytes data with reading the desired 2 bytes data.

This is used to avoid the issue which is described in
commit b4d99def

 ("r8152: remove sram_read"). The
original method always reads 4 bytes data, and it may
have problem when reading the PHY registers.

The new method is supported since RTL8153B, but it
doesn't influence the previous chips. The bits of the
byte_enable for the previous chips are the reserved
bits, and the hw would ignore them.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>