drivers: usb: Replace k_mem_block usage

The k_mem_block structure has been obsolete for some years with
a few loose ends still remaining. As its usage in the usb_dc_mcux
codebase was confined to needing its "void *" pointer field, it
was simple to enough to hoist that field to remove another loose end.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>