"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.

[Original U-Boot patch by Alexey Brodkin <abrodkin@synopsys.com>]

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>