Samples: Fix initialization discards qualifiers warnings in xusb.c

Samples: Fix initialization discards qualifiers warnings in xusb.c

xusb.c: In function ‘read_ms_winsub_feature_descriptors’:
xusb.c:542: warning: initialization discards qualifiers from pointer target type
xusb.c:543: warning: initialization discards qualifiers from pointer target type
xusb.c: In function ‘test_device’:
xusb.c:598: warning: initialization discards qualifiers from pointer target type
xusb.c:598: warning: initialization discards qualifiers from pointer target type
xusb.c:598: warning: initialization discards qualifiers from pointer target type
xusb.c:599: warning: initialization discards qualifiers from pointer target type
xusb.c:599: warning: initialization discards qualifiers from pointer target type