ARM: omap1/ams-delta: warn about failed regulator enable

The modem pm handler in the ams-delta board uses regulator_enable()
but does not check for a successful return code:

board-ams-delta.c:521:3: error: ignoring return value of 'regulator_enable', declared with attribute warn_unused_result [-Werror=unused-result]

It is not easy to propagate that return code to the callers in
uart_configure_port/uart_suspend_port/uart_resume_port, unless
we change all UART drivers, and it is unclear what those would
do with the return code.

Instead, this patch uses a runtime warning to replace the
compiletime warning. I have checked that the regulator in question
is hardcoded to a fixed-voltage GPIO regulator, and that should
never fail to get enabled if I understand the code right.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Link: https://patchwork.kernel.org/patch/8391981/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>