octeontx2-pf: Fix an IS_ERR() vs NULL bug

The otx2_mbox_get_rsp() function never returns NULL, it returns error
pointers on error.

Fixes: 34bfe0eb ("octeontx2-pf: MTU, MAC and RX mode config support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>