sparc: Pass buffer pointer all the way down to prom_{get,put}char().

sparc: Pass buffer pointer all the way down to prom_{get,put}char().

This gets us closer to being able to eliminate the use
of dynamic and stack based buffers, so that we can adhere
to the "no buffer addresses above 4GB" rule for PROM calls.

Signed-off-by: David S. Miller <davem@davemloft.net>