staging: rtl8192u: Check some memory allocation failure

If one of these memory allocations fail, a NULL pointer dereference will
occur later on.
Return -ENOMEM instead.

There is no need to free the resources already allocated, this is done
by the caller (i.e. 'rtl8192_usb_probe()') which calls
'rtl8192_usb_deleteendpoints()'.

The calling graph is:
rtl8192_usb_probe
  --> rtl8192_init
    --> rtl8192_usb_initendpoints

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>