i3c/master: Fix uninitialized variable next_addr

The variable next_addr is not initialized and is being used in a call
to i3c_master_get_free_addr as a starting point to find the next address.
Fix this by initializing next_addr to 0 to avoid an uninitialized garbage
starting address from being used.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 9ad9a52c ("i3c/master: introduce the mipi-i3c-hci driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-i3c/20201124123504.396249-1-colin.king@canonical.com
(cherry picked from commit 8e345737)