ibmvnic: parenthesize a check

Parenthesize a check to be more explicit and to fix a sparse warning
seen on some distros.

Fixes: 91dc5d25

 ("ibmvnic: fix miscellaneous checks")
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>