usb: ISO endpoint size may not be power of 2.

By this commit exception for ISO endpoints is made when it comes
to its size. ISO endpoint buffer size for nrf devices is 1023 and
may be configured with variable length size. NRFX checks is size
is chosen accordingly and it is no reason to do it in SHIM.

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>