[MIPS] Alchemy: Fix ids in Alchemy db dma device table

0 is a valid device id (DSCR_CMD0_UART0_TX), so we can't use it to mark
an empty entry in the device table. Use ~0 instead and search for id ~0
when looking for a free entry.

Signed-off-by: Wolfgang Ocker <weo@reccoware.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>