drviers: mipi_dbi: mipi_dbi_bitbang: fix 16-bit data_gpios exceed error

For a 16-bit data bus, "DT_INST_PROP_LEN(n, data_gpios)" returns 16
which is equals "MIPI_DBI_MAX_DATA_BUS_WIDTH(16)".
As a result, the assertion will always be triggered.

Use a "<=" condition for the assertion instead.

Signed-off-by: Hua Zheng <writeforever@foxmail.com>