cxgb4: remove DEFINE_SIMPLE_DEBUGFS_FILE()

We already have the DEFINE_SHOW_ATTRIBUTE. There is no need to define
such a macro, so remove DEFINE_SIMPLE_DEBUGFS_FILE. Also use the
DEFINE_SHOW_ATTRIBUTE macro to simplify some code.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>