microblaze: exception: fix return address for delay slot exceptions

According to the MicroBlaze reference manual (xilinx2021.2/ug984/page-37):
"""
If an exception is caused by an instruction in a delay slot (that is,
ESR[DS]=1), the exception handler should return execution to
the address stored in BTR instead of the normal exception return
address stored in R17.
"""

Adjust the code to print the proper return address for delay slot
exceptions.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Link: https://lore.kernel.org/r/20220213080925.1548411-4-ovidiu.panait@windriver.com


Signed-off-by: Michal Simek <michal.simek@xilinx.com>