usb: usb251xb: remove max_{power,current}_{sp,bp} properties

Remove the max_{power,current}_{sp,bp} properties of the usb251xb driver
from devicetree. This is done to simplify the dt bindings as requested
by Rob Herring in https://lkml.org/lkml/2017/2/15/1283

. If those
properties are ever needed by somebody they can be enabled again easily.

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>