spi: spi-mem: Fix a memory leak in spi_mem_dirmap_destroy()

The dirmap descriptor object allocated in spi_mem_dirmap_create is
never freed. Add a kfree(desc) in spi_mem_dirmap_destroy().

Fixes: aa167f3f ("spi: spi-mem: Add a new API to support direct mapping")
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>