clocksource/drivers/bcm2835_timer: Fix memory leak of timer

Currently when setup_irq fails the error exit path will leak the
recently allocated timer structure.  Originally the code would
throw a panic but a later commit changed the behaviour to return
via the err_iounmap path and hence we now have a memory leak. Fix
this by adding a err_timer_free error path that kfree's timer.

Addresses-Coverity: ("Resource Leak")
Fixes: 524a7f08 ("clocksource/drivers/bcm2835_timer: Convert init function to return error")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191219213246.34437-1-colin.king@canonical.com