xtensa: userspace: use ZSR_RFI_LEVEL for syscall return

When returning from syscall, we cannot use RFE by using EPC1.
As there is no EPS1, we would need to write to PS before
returning. However, this creates a situation where interrupts
are being enabled (since PS is set), and any interrupts will
overwrite EPC1 before we return (which ensures chaos as we
would be returning to the wrong address). So utilize the same
mechanism as interrupt returning by use ZSR_EPS, ZSR_EPC and
ZSR_RFI_LEVEL.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>