firmware/ice40-riscv/e1-tracer: Fix e1_tick_read()

This was broken when moving the design to non-hacked multi channel
E1.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Change-Id: I5e7ce22935609bc64333e2d5c310eb7493555a93
1 file changed