ARM: OMAP1: ams-delta: update the modem to use regulator API

After the CX20442 codec driver already takes care of enabling the codec
power for itself (commit f75a8ff6,
"ASoC: cx20442: add bias control over a platform provided regulator"),
but before dropping the old bias control method from the Amstrad Delta
ASoC sound card file, which in fact keeps the modem power always on,
even on the ASoC device close for now, extend the modem setup with a
power management callback which toggles the regulator up to the modem's
needs, reusing the previously set up regulator consumer for this. Also,
drop the MODEM_NRESET pin setup from the modem initialization procedure,
as this operation was already ineffective since patch 1/3, and not
needed because the regulator is set up as initially enabled.

Depends on patch 1/3 "ARM: OMAP1: ams-delta: set up regulator over modem
reset GPIO pin" to apply cleanly.

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>