ASoC: Intel: Move Baytrail extended fw address saving to sst_byt_boot()

We have to save the physical address of extended firmware block in the
beginning of mailbox every time when we boot the DSP firmware since that
mailbox address is re-used after DSP firmware is running. Otherwise DSP
firmware will get bogus extended firmware block address during next
DSP boot.

Fix this by moving extended firmware address saving from sst_byt_init() to
sst_byt_boot().

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>