lib: sbi_misaligned_ldst: Determine transformed instruction length correctly

If MTINST[0:0] bit is 1 then we have transformed instruction encoding
in MTINST CSR. For transformed instructions, if the MTINST[1:1] bit
is Zero then original trapped instruction was a 16bit instruction
which was converted to 32bit instruction at time of taking trap.

We should use MTINST[1:1] bit to determine correct instruction length
of transformed instruction.

This patch updates misaligned load/store emulation as-per above.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>