power: supply: sbs-battery: simplify read_read_string_data

The SBS battery implements SMBus block reads. Currently the
driver "emulates" this by doing an I2C byte read for the
length followed by an I2C block read. The I2C subsystem
actually provides a proper API for doing SMBus block reads,
which can and should be used instead. The current implementation
does not properly handle packet error checking (PEC).

This change requires, that I2C bus drivers support I2C_M_RECV_LEN
or directly provide the SMBus API to access device manufacturer
and model name.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>