ALSA: fm801: introduce macros to access the hardware

It will help to maintain HW accessors and, for example, switch from the
direct I/O to MMIO which is more convenient for PCI devices.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>