spi: Unify firmware node type checks

spi: Unify firmware node type checks

The few functions are using different approaches on how to check for
the type of firmware node. Unify them to use a modern way of it.

With that in place it becomes obvious that no need to have independent
conditionals when they are dependent and hence the code generation can
be improved a little bit (clang-18, x86_64):

  add/remove: 0/0 grow/shrink: 2/2 up/down: 16/-46 (-30)
  Total: Before=49801, After=49771, chg -0.06%

Meanwhile no functional changes intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20241208195635.1271656-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>