clk: xgene: Add missing parenthesis when clearing divider value

In the initial fix for non-zero divider shift value, the parenthesis
was missing after the negate operation. This patch adds the required
parenthesis. Otherwise, lower bits may be cleared unintentionally.

Signed-off-by: Loc Ho <lho@apm.com>
Acked-by: Toan Le <toanle@apm.com>
Fixes: 1382ea63 ("clk: xgene: Fix divider with non-zero shift value")
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>