IB/core: remove redundant check on prot_sg_cnt

prot_sg_cnt cannot be zero as a previous check on ret (from which
prot_sg_cnt is assigned) returns -ENOMEM if is it zero.  Since
it cannot be zero we can simplify the code by removing the non
-zero check on prot_sg_cnt and redundant else statement.

Detected by CoverityScan, COD#1357188 ("Logically dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>