USB: c67x00: add proper delays to HPI read/write

According to CY7C67300 specification HPI read and write cycle duration
Tcyc must be at least 6T long, where T is 1/48MHz, which is 125ns.
Without this delay fast host processor cannot write to chip registers.
Add proper ndelay to hpi_{read,write}_reg.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>