ToAHB: fix a bug where a non-flow AHB adapter lowers hbusreq (#2023)

In non-flow mode, the following sequence of events could occur:

1. a.valid goes high with a request => hbusreq goes high
2. hgrant goes high (in the same cycle as 1 or much later)
3. granted goes high causing a.ready to go high
   ... and also lowers the hbusreq cause (a.valid && !granted)
4. the output request causes hbusreq to go high again

This change shortens the behaviour of the non-flow pipeline:

1. a.valid goes high with a request => hbusreq goes high
2. hgrant goes high ***causing a.ready to go high***
3. the output request keeps hbusreq high, despite granted high