staging: gasket: Fix sparse "incorrect type in assignment" warnings.

Remove the coherent buffer __iomem cookie because the buffer is
allocated from dma_alloc_coherent().

warning: incorrect type in assignment (different address spaces)
   expected unsigned char [noderef] [usertype] <asn:2>*virt_base
   got void *[assigned] mem
warning: incorrect type in argument 3 (different address spaces)
   expected void *cpu_addr
   got unsigned char [noderef] [usertype] <asn:2>*virt_base

Signed-off-by: Laurence Rochfort <laurence.rochfort@gmail.com>
Reviewed-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>