[PATCH] powerpc: Loosen udbg_probe_uart_speed sanity checking

The checking of the baudrate in udbg_probe_uart_speed was
too tight and would cause reporting back of the default
baud rate in cases where the computed speed was valid.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>