microblaze: exception: fix unaligned data access register mask

The correct mask for getting the source/destination register from ESR in
the case of an unaligned access exception is 0x3E0. With this change, a
dummy unaligned store produces the expected info:
"""
>> swi r5, r0, 0x111

 ...
 Hardware exception at 0x111 address
 Unaligned data access exception
 Unaligned word access
 Unaligned store access
 Register R5
 Return address from exception 0x7f99dfc
 ### ERROR ### Please RESET the board ###
"""

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Link: https://lore.kernel.org/r/20220213080925.1548411-6-ovidiu.panait@windriver.com


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