Correct computation of vector offsets for EXCP_EXT_INTERRUPT.
For instance, if Cause.IV is 0 the vector offset should be 0x180.

Simplify the finding vector number logic for the Vectored Interrupts.
Signed-off-by: NYongbok Kim <yongbok.kim@imgtec.com>
Reviewed-by: NLeon Alrae <leon.alrae@imgtec.com>
[leon.alrae@imgtec.com: cosmetic changes]
Signed-off-by: NLeon Alrae <leon.alrae@imgtec.com>