spi: mt7621: Use devm_spi_register_controller()

Now that clk_disable_unprepare(clk) is handled with a managed resource,
we can use devm_spi_register_controller() and axe the .remove function.

The order between spi_unregister_controller() and clk_disable_unprepare()
is still the same.
(see commit 46b5c4fb ("spi: mt7621: Don't leak SPI master in probe
error path") to see why it matters)

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/b7698e198acc998d99e7e7c895a2910f14f89443.1661599671.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Mark Brown <broonie@kernel.org>