pc-bios/s390-ccw/bootmap: Silence compiler warning from Clang

When compiling the s390-ccw bios with Clang, the compiler complains:

 pc-bios/s390-ccw/bootmap.c:302:9: warning: logical not is only applied
  to the left hand side of this comparison [-Wlogical-not-parentheses]
    if (!mbr->dev_type == DEV_TYPE_ECKD) {
        ^              ~~

The code works (more or less by accident), since dev_type can only be
0 or 1, but it's better of course to use the intended != operator here
instead.

Fixes: 5dc739f3 ("Allow booting in case the first virtio-blk disk is bad")
Message-Id: <20210421163331.358178-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>