ibcore: Fix an array index check

The array ib_mad_mgmt_class_table.method_table has MAX_MGMT_CLASS
(80) elements. Hence compare the array index with that value instead
of with IB_MGMT_MAX_METHODS (128). This patch avoids that Coverity
reports the following:

Overrunning array class->method_table of 80 8-byte elements at element index 127
(byte offset 1016) using index convert_mgmt_class(mad_hdr->mgmt_class)
(which evaluates to 127).

Linux commit:
2fe2f378dd45847d2643638c07a7658822087836

Submitted by: hselasky@
Approved by: hselasky (mentor)
MFC after: 1 week
Sponsored by: Mellanox Technologies