hwmon: (dimmtemp) Fix bitmap handling

Building arm:allmodconfig may fail with the following error.

In function 'fortify_memcpy_chk',
    inlined from 'bitmap_copy' at include/linux/bitmap.h:261:2,
    inlined from 'bitmap_copy_clear_tail' at include/linux/bitmap.h:270:2,
    inlined from 'bitmap_from_u64' at include/linux/bitmap.h:622:2,
    inlined from 'check_populated_dimms' at
	drivers/hwmon/peci/dimmtemp.c:284:2:
include/linux/fortify-string.h:344:25: error:
	call to '__write_overflow_field' declared with attribute warning:
	detected write beyond size of field (1st parameter)

The problematic code is
	bitmap_from_u64(priv->dimm_mask, dimm_mask);

dimm_mask is declared as u64, but the bitmap in priv->dimm_mask is only
24 bit wide. On 32-bit systems, this results in writes over the end of
the bitmap.

Fix the problem by using u32 instead of u64 for dimm_mask. This is
currently sufficient, and a compile time check to ensure that the number
of dimms does not exceed the bit map size is already in place.

Fixes: 73bc1b88 ("hwmon: peci: Add dimmtemp driver")
Cc: Iwona Winiarska <iwona.winiarska@intel.com>
Reviewed-by: Iwona Winiarska <iwona.winiarska@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>