ARM: OMAP1: ams-delta: assign MODEM IRQ from GPIO descriptor

Don't request MODEM IRQ GPIO by its global number in
ams_delta_modem_init().  Instead, obtain its GPIO descriptor
and assign related IRQ to the MODEM.  Do that from
omap_gpio_deps_init(), where the chip is already looked up.  Then, in
ams_delta_modem_init(), just check for the IRQ number having been
already assigned.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>