drivers: mipi-dbi-spi: rename underscored macro names

Rename _XFR_.*BITS and _WRITE_ONLY_ABSENT according to MISRA-C standard
rules 21.1 and 21.2 (Zephyr rules 121 and 122). For details see Zephyr
coding guidelines in table "Main rules".

Signed-off-by: Stephan Linz <linz@li-pro.net>