clk:at91: Fix memory leak in of_at91_clk_master_setup()

cppcheck detected following error
[clk-master.c:245]: (error) Memory leak: characteristics

The original code forgot to free characteristics when
irq_of_parse_and_map() failed.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>