staging: iio: isl29028: remove enable flag from isl29028_enable_proximity()

isl29028_enable_proximity() has a boolean argument named enable. This
function is only called once and the enable flag is set to true in that
call. This patch removes the enable parameter from that function.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>