mfd: rt5033: Fix chip revision readout

mfd: rt5033: Fix chip revision readout

After reading the data from the DEVICE_ID register, mask 0x0f needs to be
applied to extract the revision of the chip [1].

The other part of the DEVICE_ID register, mask 0xf0, is a vendor identification
code. That's how it is set up at similar products of Richtek, e.g. RT9455 [2]
page 21 top.

[1] https://github.com/msm8916-mainline/linux-downstream/blob/GT-I9195I/drivers/mfd/rt5033_core.c#L484
[2] https://www.richtek.com/assets/product_file/RT9455/DS9455-00.pdf

Signed-off-by: Jakob Hauser <jahau@rocketmail.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/9a98521ffdf76851d5d344afa6ce65f692ecc024.1684182964.git.jahau@rocketmail.com