staging: slicoss: Remove dma_addr_t cast compilation warnings

Eliminate some warnings by casting to unsigned long before
casting a dma_addr_t value to a pointer.

btw:

Does slicoss always work on x86-32?
Is a pshmem guaranteed to be accessible
by a 32 bit address?

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>