disk: Add .disk_init() to firmware struct

disk: Add .disk_init() to firmware struct

Each firmware has its own method of initialising a disk so allow each
firmware backend to implement a separate disk_init() function.

Make all the assembly callers of fs_init() jump through pm_fs_init(),
which allows us to restrict the com32sys_t arguments to only those
callers that really need it, e.g. the .asm files.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>