fix bnop displacement decoding in header based fetch packet

bnop displacement should be shifted left by 1 within compact instruction
packets (instead of 2 in non-header-based packets). See SPRUFE8B p. 167.

The infrastructure was already in place but the corresponding 32bit bnop
opcode incorrectly used 'pcrel' instead of 'pcrel_half' one.

This has been verified by disassembling working code and checking disassembly
output. Would worth a test case in the gas test-suite and cross-check against
Ti SDK tools.