flash_fp_mcu: Make exit status more distinctive
flash_fp_mcu: Make exit status more distinctive

This helps with parsing errors in tast.

BRANCH=none
BUG=none
TEST=# dut1 is a dratini device
     scp util/flash_fp_mcu dut1:/usr/local/bin/flash_fp_mcu
     ssh dut1 flash_fp_mcu --hello; echo "Status $?"
     # Should exit with 0
     ssh dut1 touch /tmp/exists
     ssh dut1 flash_fp_mcu -r /tmp/exists; echo "Status $?"
     # Should exit with 5
TEST=# Really stared at the code to make sure it looks right.

Change-Id: I1f525b99334ea3ff866fca35112119604866f939
Signed-off-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2862548
Commit-Queue: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec