sky2: fix ram buffer allocation settings

Different chipsets have different amount of ram buffer (some have none),
so need to make sure that driver does proper setup for all cases from 0 on
to 48K, in units of 1K.

This is a backport of the code from 2.6.19 or later

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>