atheepmgr: avoid libpciaccess dependency

HAVE_LIBPCIACCESS that is currently passed through MAKE_VARS to disable
building with libpciaccess can't be set through the environment.
Instead, use CONFIG_CON_PCI, which can be passed through the environment
and will disable libpciaccess.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>