ALSA: ctxfi: sparse warning

fixed sparse warning of incorrect type (different address spaces) in
cthw20k1.c and cthw20k2.c which was being actually caused as mem_base
was of the type unsigned long.

Again as mem_base was previously unsigned long , so it required many
typecasts in the code to convert interger to pointer.

Now after giving the correct type of mem_base as void __iomem *
we can also remove those typecasts maintaining the same functionality
and logic of the code.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>