USB: EHCI: fix bug in scheduling periodic split transfers
This patch (as1654) fixes a very old bug in ehci-hcd, connected with
scheduling of periodic split transfers. The calculations for
full/low-speed bus usage are all carried out after the correction for
bit-stuffing has been applied, but the values in the max_tt_usecs
array assume it hasn't been. The array should allow for allocation of
up to 90% of the bus capacity, which is 900 us, not 780 us.
The symptom caused by this bug is that any isochronous transfer to a
full-speed device with a maxpacket size larger than about 980 bytes is
always rejected with a -ENOSPC error.