IB/qib: fix false-postive maybe-uninitialized warning

aarch64-linux-gcc-7 complains about code it doesn't fully understand:

drivers/infiniband/hw/qib/qib_iba7322.c: In function 'qib_7322_txchk_change':
include/asm-generic/bitops/non-atomic.h:105:35: error: 'shadow' may be used uninitialized in this function [-Werror=maybe-uninitialized]

The code is right, and despite trying hard, I could not come up with a version
that I liked better than just adding a fake initialization here to shut up the
warning.

Fixes: f931551b

 ("IB/qib: Add new qib driver for QLogic PCIe InfiniBand adapters")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>