lan: channel: Fix set alert on/off

From IPMI Spec, Chapter 22.22 Set Channel Access Command
Table 22, Set Channel Access Command

Byte#2, Bit#5 is "PEF Alerting Enable/Disable"
And the bit value:
    0b = enable PEF Alerting
    1b = disable PEF Alerting on this channel

In current code, alert "on" set Bit#5 to 1 and alert "off" set Bit#5 to
0, it's straightforward but just opposite of IPMI spec bit definition.

Resolves ipmitool/ipmitool#247

Reported-by: Ryan Fang <Ryan.Fang@quantatw.com>
Signed-off-by: Alexander Amelkin <alexander@amelkin.msk.ru>