dp83640: correct the periodic output frequency

The phyter driver incorrectly feeds the value of the period into what
is in fact a pulse width register, resulting in the actual period
being twice the dialed value. This patch fixes the issue and renames a
variable to make the code at bit more clear.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>