usb: musb: blackfin: fix build break

commit cc92f681 (usb: musb: Populate new IO
functions for blackfin) added a typo which
prevented MUSB's blackfin glue layer from being
built. Due to lack of tests and compilers for
that architecture, the typo ended up being
merged and causing a build regression.

Fix that here

Cc: Tony Lindgren <tony@atomide.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>