gpio: omap: prepare and unprepare the debounce clock

Replace the clk_enable()s with a clk_prepare_enable() and
the clk_disables()s with a clk_disable_unprepare()

This never showed issues due to the OMAP platform code (hwmod)
leaving these clocks in clk_prepare()ed state by default.

Reported-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Acked-by: Javier Martinez Canillas <javier@dowhile0.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>