usb: gadget: Remove reference to is_dualspeed from sysfs.

This commit removes the /sys/devices/platform/<UDC>/udc/<UDC>/is_dualspeed
file and is_dualspeeed line from /sys/devices/platform/ci13xxx_*/udc/device
file.

The is_dualspeed file is superseded by maximum_speed in the same directory
and is_dualspeed line in device file is superseded by max_speed line in
the same file.

The maximum_speed/max_speed specifies maximum speed supported by UDC.
To check if dualspeeed is supported, check if the value is >= 3.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>