tests: drivers: spi: spi_loopback: fix skip for unsupported

[nordic-piks]

Use ztest_test_skip to stop further execution.

[nordic-piks]

After #87838
testing fails like this:

START - test_spi_word_size_16
E: Word sizes other than 8 bits are not supported
Spi config invalid for this controller - skip

    Assertion failed at ../../../../../../../../../zephyr/tests/drivers/spi/spi_loopback/src/spi.c:628: spi_loopback_test_word_size: (memcmp(compare_data, rx_buffer, buffer_size) is true)
16-bit word buffer contents are different
 FAIL - test_spi_word_size_16 in 0.029 seconds

This add proper skip, which ends execution of particular test case.

@decsny FYI

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[nordic-piks]

Closing in favor of #90321