net: qualcomm: rmnet: Remove invalid condition while stamping mux id

rmnet devices cannot have a mux id of 255. This is validated when
assigning the mux id to the rmnet devices. As a result, checking for
mux id 255 does not apply in egress path.

CRs-Fixed: 2156182
Change-Id: I0e6c02f9ea58f347b70991c9a5c6e5881d552a01
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>