power: supply: max17040: Change register transaction length from 8 bits to 16 bits

According to the datasheet, MAX17040 has six 16-bit registers.
Register reads and writes are only valid if all 16 bits are transferred.
Any write command that is terminated early is ignored.
So it's better to change register transacton length from 8 bits to 16 bits.

Signed-off-by: Liu Xiang <liu.xiang6@zte.com.cn>
Signed-off-by: Sebastian Reichel <sre@kernel.org>