mmc: ca_dw_mmc: Misc cleanup of driver

- Rename DT compatible name
- Remove uneccessary if-statement to support 8-bit buswidth
- Remove redundant error msg
- Use symbolic constants in switch statement

Signed-off-by: Arthur Li <arthur.li@cortina-access.com>
Signed-off-by: Alex Nemirovsky <alex.nemirovsky@cortina-access.com>
CC: Peng Fan <peng.fan@nxp.com>
CC: Jaehoon Chung <jh80.chung@samsung.com>
CC: Tom Rini <trini@konsulko.com>