sparc64: perf: Use UREG_FP rather than UREG_I6

perf walks userspace callchains by following frame pointers. Use the
UREG_FP macro to make it clearer that the %fp is being used.

Signed-off-by: David Ahern <david.ahern@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>