sharpsl_pm.c code tries to read battery state very early during
resume, but those battery meters are connected on SPI and that's only
resumed way later.

Replace the check with simple checking of battery fatal signal, that
actually works at this stage.
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Stanislav Brabec <utx@penguin.cz>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>