In case the data was received in clk_ref_i domain, the flush_p1_i
was also received in the clk_ref_i domain. However, it was used
in an FSM that works in the clk_sys_i domain. This could cause
problems, for example missing the flash_p1_i pulses, thus frames
not being sent when requested. It was easily seen in the
spec_fixed_latency-demo testbench. In principle, the tx_flush_p1_i
does not need to come with data, can be asynchronous to data.
It should be a pulse, yet it can happen that it is constantly high
(see Tom's testbench of fixed-latency mode). Thus in cross-domain
use case :
1. first the pulse is extended to to cycles
2. thanks to this, the gc_sync_ffs module can be used to
   pass the signal to clk_sys clock domain, whether it is a
   pulse or not.