pwm: sun4i: Fix inconsistent IS_ERR and PTR_ERR

Fix inconsistent IS_ERR and PTR_ERR in sun4i_pwm_probe().

The proper pointers to be passed as arguments are pwm->clk and pwm->bus_clk.

This bug was detected with the help of Coccinelle.

Fixes: b8d74644 ("pwm: sun4i: Prefer "mod" clock to unnamed")
Fixes: 5b090b43 ("pwm: sun4i: Add an optional probe for bus clock")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>