ide: Remove unneeded null pointer check

With bm == NULL, other code in the same function would crash.

This bug was reported by cppcheck:
hw/ide/pci.c:280: error: Possible null pointer dereference: bm

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
