hwmon: (pwm-fan) Fix RPM calculation

hwmon: (pwm-fan) Fix RPM calculation

[ Upstream commit fd8feec665fef840277515a5c2b9b7c3e3970fad ]

To convert the number of pulses counted into an RPM estimation, we need
to divide by the width of our measurement interval instead of
multiplying by it. If the width of the measurement interval is zero we
don't update the RPM value to avoid dividing by zero.

We also don't need to do 64-bit division, with 32-bits we can handle a
fan running at over 4 million RPM.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Link: https://lore.kernel.org/r/20201111164643.7087-1-pbarker@konsulko.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>