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.
The origin method always reads 4 bytes data, and it may have problem
when reading the PHY regiters.
The new method is supported since RTL8152B, 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@xxxxxxxxxxx>