staging/mei: don't check if list is empty before looping

1. No need to check if a list is empty before list_for_each_ looping as
this is already checked by loop stopping conditional.

The side effect is reduced indentation depth
from:
	if (!list_empty)
		list_for_each()
to:
	list_for_each()

2. drop cb_ prefix from cl_pos, cl_next variables used in list_for_each
loops. The code is more compact and readable

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>