musb_hdrc: cleanup, mostly related to CPPI

 - Remove #ifdefs in favor of new is_cppi_enabled() macro.
 - Trim some tx-start logic, inlining it and removing MGC_{Read,Write}Csr16()
 - Remove remnant of bogus/non-existent host-side ISO bitfield.

For upstream merging, more such #ifdeffery still needs to be removed.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>