staging: btmtk_usb: check for a valid io_buf pointer

assigned to retval of kmalloc but not checked whether the allocation failed or not,
fail the registering if allocation fail

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>