x86: Mark device_nb as static and fix NULL noise

This sparse warning:

  arch/x86/kernel/amd_iommu.c:1195:23: warning: symbol 'device_nb' was not declared. Should it be static?

triggers because device_nb is global but is only used in a
single .c file. change device_nb to static to fix that - this
also addresses the sparse warning.

This sparse warning:

  arch/x86/kernel/amd_iommu.c:1766:10: warning: Using plain integer as NULL pointer

triggers because plain integer 0 is used in place of a NULL
pointer. change 0 to NULL to fix that - this also address the
sparse warning.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Joerg Roedel <joerg.roedel@amd.com>
LKML-Reference: <1246458194.6940.20.camel@hpdv5.satnam>
Signed-off-by: Ingo Molnar <mingo@elte.hu>