drivers: spi: update error type to skip word_size testcases

Workaround to avoid getting the -EFAULT (14) error when running
the test_spi_word_size_x (x = 7, 9, 24) test cases:
Since these testcase configurations aren't supported, update
the error type to ENOTSUP to allow skipping the test cases using ztest.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>