musb_hdrc, remove MGC_ReadCsr16()

More camelcase fixes:  remove MGC_ReadCsr16.

Note that these changes are not just code cleanup, but they're also
code shrinks and speedups because the endpoint access routines no
longer need to calclate the offsets they're using...

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