usb: musb: fix order of conditions for assigning end point operations

Currently we always assign one of the two common implementations of
ep_offset and ep_select operations, overwriting any platform-specific
implementations.

Fixes: d026e9c7 ("usb: musb: Change end point selection to use ...")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Felipe Balbi <balbi@ti.com>