RDMA/cxgb4: Remove the obsolete kernel module option 'c4iw_debug'

This patch avoids that building the cxgb4 module with W=1 triggers
a complaint about a local variable that has not been declared static.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Steve Wise <swise@opengridcomputing.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>