mmc: moxart: fix 4-bit bus width and remove 8-bit bus width

According to the datasheet [1] at page 377, 4-bit bus width is turned on by
bit 2 of the Bus Width Register. Thus the current bitmask is wrong: define
BUS_WIDTH_4 BIT(1)

BIT(1) does not work but BIT(2) works. This has been verified on real MOXA
hardware with FTSDC010 controller revision 1_6_0.

The corrected value of BUS_WIDTH_4 mask collides with: define BUS_WIDTH_8
BIT(2). Additionally, 8-bit bus width mode isn't supported according to the
datasheet, so let's remove the corresponding code.

[1]
https://bitbucket.org/Kasreyn/mkrom-uc7112lx/src/master/documents/FIC8120_DS_v1.2.pdf

Fixes: 1b66e94e

 ("mmc: moxart: Add MOXA ART SD/MMC driver")
Signed-off-by: Sergei Antonov <saproj@gmail.com>
Cc: Jonas Jensen <jonas.jensen@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220907205753.1577434-1-saproj@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>