USB: ice40-hcd: Fix configuration loading frequency usage

The maximum operating frequency during configuration loading is
25MHz. This is hard coded in the driver. If the maximum supported
frequency of the SPI device is less than 25MHz, the configuration
loading is failing. Fix this issue by limiting the configuration
loading frequency to the maximum supported frequency.

Change-Id: I854ebcdd77e4708883dc03175021e1ef1e9e63a9
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>