bus: sunxi-rsb: Return correct data when mixing 16-bit and 8-bit reads

When doing a 16-bit read that returns data in the MSB byte, the
RSB_DATA register will keep the MSB byte unchanged when doing
the following 8-bit read. sunxi_rsb_read() will then return
a result that contains high byte from 16-bit read mixed with
the 8-bit result.

The consequence is that after this happens the PMIC's regmap will
look like this: (0x33 is the high byte from the 16-bit read)

% cat /sys/kernel/debug/regmap/sunxi-rsb-3a3/registers
00: 33
01: 33
02: 33
03: 33
04: 33
05: 33
06: 33
07: 33
08: 33
09: 33
0a: 33
0b: 33
0c: 33
0d: 33
0e: 33
[snip]

Fix this by masking the result of the read with the correct mask
based on the size of the read. There are no 16-bit users in the
mainline kernel, so this doesn't need to get into the stable tree.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>