gatppp: fix ppp protocol-reject constructing error

gatppp: fix ppp protocol-reject constructing error

In PPP Protocol-Reject package the 'rejected data field' should be a copy
of the rejected package, the copy starts from the data field. besides,
Protocol-Reject package include a 'rejected protocol field', it is the
same as the protocol field of the rejected package.

Protocol-Reject package structure is:

	| PPP_header | rejected protocol | rejected data

The rejected package structure is:
	| Addr | Control | protocol | data

So the Protocol-Reject package data field is copied from the 3rd byte of
the rejected package.