drivers/isdn: Drop unnecessary NULL test

The result of container_of should not be NULL.  In particular, in this case
the argument to the enclosing function has passed though INIT_WORK, which
dereferences it, implying that its container cannot be NULL.

A simplified version of the semantic patch that makes this change is as
follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@@
identifier fn,work,x,fld;
type T;
expression E1,E2;
statement S;
@@

static fn(struct work_struct *work) {
  ... when != work = E1
  x = container_of(work,T,fld)
  ... when != x = E2
- if (x == NULL) S
  ...
}
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Karsten Keil <keil@b1-systems.de>

drivers/isdn/hisax/hfc_sx.c

@@ -1255,8 +1255,6 @@ hfcsx_bh(struct work_struct *work)
 		container_of(work, struct IsdnCardState, tqueue);
 	u_long flags;
 
-	if (!cs)
-		return;
 	if (test_and_clear_bit(D_L1STATECHANGE, &cs->event)) {
 		if (!cs->hw.hfcsx.nt_mode)
 			switch (cs->dc.hfcsx.ph_state) {