isdn: use constants instead of magicnumbers

This patch changes instances of magic numbers like 4 and 8 to equivalent
constants.

The Coccinelle semantic patch used for making the change is as follows:

// <smpl>
@r@
type T;
T E;
identifier fld;
identifier c;
@@

E->fld & c

@s@
constant C;
identifier r.c;
@@

#define c C

@@
r.T E;
identifier r.fld;
identifier r.c;
constant s.C;
@@

 E->fld &
- C
+ c
// </smpl>

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>