gpio/omap: move bank->dbck initialization to omap_gpio_mod_init()

Since the bank->dbck initialization in a one time operation there
is no need to keep this within gpio_debounce(). Therefore, moving
clk_get(bank->dbck) to omap_gpio_mod_init(). Since the value of
bank->dbck would be NULL at the beginning, this check has been
removed.

Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Reported-by: Paul Walmsley <paul@pwsan.com>
Reviewed-by: Jon Hunter <jon-hunter@ti.com>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Cousson, Benoit <b-cousson@ti.com>
Reviewed-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>