iw_cxgb4: make pointer reg_workq static

The pointer reg_workq is local to the source and does not need to be
in global scope, so make it static.

Cleans up sparse warning:
drivers/infiniband/hw/cxgb4/device.c:69:25: warning: symbol 'reg_workq'
was not declared. Should it be static?

Fixes: 1c8f1da5 ("iw_cxgb4: Fix possible circular dependency locking warning")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>