The current code has the possibility that we could set reg_addr
or reg_ctrl and then increment reg_addr in the same cycle, resulting
in some long timing paths. Rearrange the code to make it clear
that we are not trying to add an auto-increment to data from
outside the module; in any given cycle we either set one of
reg_addr and reg_ctrl, or we possibly increment reg_addr.
Signed-off-by: Paul Mackerras <firstname.lastname@example.org>