ALSA: ctxfi - Remove PAGE_SIZE limitation

Remove the limitation of PAGE_SIZE to be 4k by defining the own
page size and macros for 4k.  8kb page size could be natively supported,
but it's disabled right now for simplicity.

Also, clean up using upper_32_bits() macro.

Signed-off-by: Takashi Iwai <tiwai@suse.de>