package/spidev_test: fix version depending on kernel headers version

Fixes:

  - http://autobuild.buildroot.net/results/a5c008aaff40f4851ffa36439c7340b6243a4842



  .../build/spidev_test-5.8/spidev_test.c:132:13: error: 'SPI_TX_OCTAL' undeclared (first use in this function); did you mean 'SPI_TX_DUAL'?
    132 |  if (mode & SPI_TX_OCTAL)
        |             ^~~~~~~~~~~~
        |             SPI_TX_DUAL

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>