rndis: remove private wrapper of __constant_cpu_to_le32

Use cpu_to_le32 directly as it handles constant folding now, replace direct
uses of __constant_cpu_to_{endian} as well.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>