staging: slicoss: avoid PCI write posting

Introduce the function slic_flush_write() which reads from the HOSTID
register and can be used to avoid PCI write posting. Use the function at
several critical places in the code.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>