clk: sunxi: factors: fix off-by-one masks

The previous code would generate one bit too long masks, and was
needlessly complicated. This patch replaces it by simpler code that can
generate the masks correctly.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>