pc-bios/s390-ccw: size_t should be unsigned

pc-bios/s390-ccw: size_t should be unsigned

"size_t" should be an unsigned type according to the C standard.
Thus we should also use this convention in the s390-ccw firmware to avoid
confusion. I checked the sources, and apart from one spot in libc.c, the
code should all be fine with this change.

Buglink: https://bugs.launchpad.net/qemu/+bug/1753437
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: Collin Walling <walling@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>