ath10k: do not check if reset is NULL

Since reset_control_get() never returns NULL, we can use IS_ERR()
instead of IS_ERR_OR_NULL().  The return statements can be simpler
as well.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>