net: designware: Respect "bus mode" register contents on SW reset

net: designware: Respect "bus mode" register contents on SW reset

"bus mode" register contains lots of fields and some of them don't
expect to be written with 0 (zero). So since we're only interested in
resetting MAC (which is done with setting the least significant bit of
this register with "0") I believe it's better to modify only 1 bit of
the register.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Vipin Kumar <vipin.kumar@st.com>
Patch: 277864