usb: f_qdss: Remove usb_request::num_mapped_sgs access

The function drivers have no business accessing usb_request::num_mapped_sgs
field. The UDC will use this field to cache the sgs mapped for a given
request.

Change-Id: I6a8c8c2e52a36caedfe977f517b4c04b1ad0045c
Signed-off-by: Pavankumar Kondeti <quic_pkondeti@quicinc.com>