clk: pxa: fix NULL pointer dereference in pxa3xx_clk_update_accr

sparse points out an embarrasing bug in an older patch of mine,
which uses the register offset instead of an __iomem pointer:

drivers/clk/pxa/clk-pxa3xx.c:167:9: sparse: sparse: Using plain integer as NULL pointer

Unlike sparse, gcc and clang ignore this bug and fail to warn
because a literal '0' is considered a valid representation of
a NULL pointer.

Fixes: 3c816d95

 ("ARM: pxa: move clk register definitions to driver")
Cc: stable@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202305111301.RAHohdob-lkp@intel.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230511105845.299859-1-arnd@kernel.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>