IB: find_first_zero_bit() takes unsigned pointer

Fix sparse warning

    drivers/infiniband/core/device.c:142:6: warning: incorrect type in argument 1 (different signedness)
    drivers/infiniband/core/device.c:142:6:    expected unsigned long const *addr
    drivers/infiniband/core/device.c:142:6:    got long *[assigned] inuse

by making the local variable inuse unsigned.  Does not affect generated
code at all.

Signed-off-by: Roland Dreier <rolandd@cisco.com>