HSI: cmt_speech: Pass a pointer to virt_to_page()

A pointer into virtual memory is represented by a (void *)
not an u32, so the compiler warns:

drivers/hsi/clients/cmt_speech.c:1092:35: warning: passing argument
  1 of 'virt_to_pfn' makes pointer from integer without a cast
  [-Wint-conversion]

Fix this with an explicit cast.

Cc: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>