fw_cfg: fix FW_CFG_BOOT_DEVICE update on ppc and sparc

On ppc, sparc, and sparc64, the value of the FW_CFG_BOOT_DEVICE 16bit
fw_cfg entry is repeatedly modified from a series of callbacks, which
currently results in the previous value's dynamically allocated memory
being leaked.

This patch switches updating to the new fw_cfg_modify_i16() call, which
does not cause memory leaks.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
