libertas: fix pointer bugs for PS_MODE commands

struct cmd_ds_802_11_ps_mode
contains the command header and a pointer to it was
initialized with data points to the body which leads to
mis-interpretation of the cmd_ds_802_11_ps_mode.action member.
cmd[0] contains the header, &cmd[1] points beyond that.
cmdnode->cmdbuf is a pointer to the command buffer
This piece of code was unused since power saving was
not enabled.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>