net: mvpp2: debugfs: remove redundant parameter check in three functions

As per the comment above debugfs_create_dir(), it is not expected to
return an error, so an extra error check is not needed.
Drop the return check of debugfs_create_dir() in
mvpp2_dbgfs_c2_entry_init(), mvpp2_dbgfs_flow_tbl_entry_init()
and mvpp2_dbgfs_cls_init().

Signed-off-by: Minjie Du <duminjie@vivo.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/20230717025538.2848-1-duminjie@vivo.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>