[XTENSA] Add missing a2 register restore in register spill routine

Register a2 is saved in depc but wasn't getting restored before
returning from _spill_registers when there weren't any registers
to spill. The mask to cut the top bit from the rotated WINDOWMASK
register was also one bit short.

Signed-off-by: CHris Zankel <chris@zankel.net>

diff --cc arch/xtensa/kernel/entry.S
Simple merge