When the clock is enabled, check if there is an error. Otherwise
clk_get_rate() can be called without enabled clock.

Found by Linux Driver Verification project (linuxtesting.org).

Fixes: 0814e8d5 ("sc16is7xx: enable the clock")
Signed-off-by: Stefan Potyra <Stefan.Potyra@elektrobit.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>