Fix Intel Dollar Cove TI battery driver 32-bit build error

Fix Intel Dollar Cove TI battery driver 32-bit build error

The driver is doing a 64-bit divide, rather than using the proper
helpers, causing link errors on i386 allyesconfig builds:

  x86_64-linux-ld: drivers/power/supply/intel_dc_ti_battery.o: in function `dc_ti_battery_get_voltage_and_current_now':
  intel_dc_ti_battery.c:(.text+0x5c): undefined reference to `__udivdi3'
  x86_64-linux-ld: intel_dc_ti_battery.c:(.text+0x96): undefined reference to `__udivdi3'

and while fixing that, fix the double rounding: keep the timing
difference in nanoseconds ('ktime'), and then just convert to usecs at
the end.

Not because the timing precision is likely to matter, but because doing
it right also makes the code simpler.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Cc: Hans de Goede <hansg@kernel.org>
Cc: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>