USB: musb: use right poll limit for low speed devices

Remove wrongly applied upper limit on the interrupt transfer
interval for low speed devices (not much of an error per se,
according to USB specs).

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>