ath9k: fix an IS_ERR() vs NULL check

The devm_kmemdup() function doesn't return error pointers, it returns
NULL on error.

Fixes: eb3a97a6 ("ath9k: fetch calibration data via nvmem subsystem")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211011123533.GA15188@kili