cifs: smb2 commands can not be negative, remove confusing check

As Coverity points out le16_to_cpu(midEntry->Command) can not be
less than zero.

Detected by CoverityScan, CID#1438650 ("Macro compares unsigned to 0")

Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>