iw_cxgb4: make pointer reg_workq static

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: 1c8f1da5d851 ("iw_cxgb4: Fix possible circular dependency locking warning")
Signed-off-by: Colin Ian King <[email protected]>
Reviewed-by: Leon Romanovsky <[email protected]>
Reviewed-by: Steve Wise <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>