clk-gate: fix bit # check in clk_register_gate()

In case CLK_GATE_HIWORD_MASK flag is passed to clk_register_gate(), the bit #
should be no higher than 15, however the corresponding check is obviously off-
by-one.

Fixes: 04577994 ("clk: gate: add CLK_GATE_HIWORD_MASK")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>