Staging: rts_pstor: s/rtsx_alloc_dma_buf/kmalloc/

Replace rtsx_alloc_dma_buf() with kfree() back,
chip parameter is not used; remove casts.

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>