usb: gsi: Mask the read/write apis for dpl_ctrl node

DPL daemon from userspace is writing control message
to dpl_ctrl node. But since the notify_req for DPL
is not supported in the driver it is resulting in a
crash. Mask the read/write api's for dpl_ctrl node
to fix this.

Change-Id: I2dc67bee0f83107b90e8754a3d10b2e128ce3937
Signed-off-by: Elson Roy Serrao <eserrao@codeaurora.org>