USB: ice40-hcd: Prevent SPI runtime suspend during power-up

The chip select pin needs to be temporarily overridden during
bridge power-up to enable the slave mode. Prevent SPI runtime
suspend for exclusive chip select pin access. Otherwise chip
select is driven high during SPI runtime suspend and bridge
enters Master mode.

Change-Id: I97977567abb366bed883afba58a2f06430c79628
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>