x86/mm: Clean up types in xlate_dev_mem_ptr()

Pavel Machek reported the following compiler warning on
x86/32 CONFIG_HIGHMEM64G=y builds:

  arch/x86/mm/ioremap.c:344:10: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Clean up the types in this function by using a single natural type for
internal calculations (unsigned long), to make it more apparent what's
happening, and also to remove fragile casts.

Reported-by: Pavel Machek <pavel@ucw.cz>
Cc: jgross@suse.com
Cc: roland@purestorage.com
Link: http://lkml.kernel.org/r/20150416080440.GA507@amd


Signed-off-by: Ingo Molnar <mingo@kernel.org>