pc-bios/s390-ccw: Fix the cc-option macro in the Makefile

pc-bios/s390-ccw: Fix the cc-option macro in the Makefile

The cc-option macro is not doing what it should - compared with the
original from the rules.mak file that got removed with commit
660f793093 ("Makefile: inline the relevant parts of rules.mak"),
the arguments got changed and thus the macro is rather doubling
the QEMU_CFLAGS than adding the flag that should be tested.

Message-Id: <20210502174836.838816-3-thuth@redhat.com>
Fixes: 22fb2ab096 ("pc-bios/s390-ccw: do not use rules.mak")
Signed-off-by: Thomas Huth <thuth@redhat.com>