Revert "power: supply: sbs-battery: simplify read_read_string_data"

The commit is a nice cleanup, but breaks booting on exynos5 based
chromebooks. It's seems to come down to exynos5's i2c driver not
implementing I2C_FUNC_SMBUS_READ_BLOCK_DATA. It's not yet clear
why that breaks boot / massively slows it down when userspace
starts, so revert the problematic patch.

This reverts commit c4b12a2f.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>