power/reset: at91-poweroff: Fix error handling and other compiler warnings

at91_poweroff_get_wakeup_mode can return a negative error code and should
therefore not return an enum type. Similar, its result should not be
assigned to an enum type. Otherwise, the returned value is never negative,
resulting in a compiler warning and a missed error condition, which in turn
results in writing bad values into a chip register.

Also fix other compiler warnings which can be easily avoided.

drivers/power/reset/at91-poweroff.c:74:24:
	warning: type qualifiers ignored on function return type
drivers/power/reset/at91-poweroff.c:74:24:
	warning: no previous prototype for 'at91_poweroff_get_wakeup_mode'
drivers/power/reset/at91-poweroff.c:83:16:
	warning: comparison between signed and unsigned integer expressions
drivers/power/reset/at91-poweroff.c:97:2:
	warning: comparison of unsigned expression < 0 is always false

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>