fsi: sbefifo: Bump max command length

Otherwise cronus putmem fails istep and BML fails to upload skiboot

To do that, we still use our one-page command buffer for small commands
for speed, and for anything bigger, with a limit of 1MB plus a page,
we vmalloc a temporary buffer.

The limit was chosen because Cronus will break up any data transfer
into 1M chunks (the extra page is for the command header).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>