ARCv2: entry: early return from exception need not clear U & DE bits

Exception handlers call FAKE_RET_FROM_EXCPN to
 - clear AE bit: drop down from exception active to pure kernel mode
   allowing further excptions
 - set IE bit: re-enable interrupts

It additionally also clears U bit (user mode) and DE bit (delay slot
execution) which is redundant as hardware does that already on any taken
exception. Morevoer the current software clearing is bogus anyways as
the KFLAG instruction being used for purpose can't possibly write those
bits anyways.

So don't pretend to clear them.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
[vgupta: rewrote changelog]