cnss2: Initialize plat_priv during bus_init

cnss_usb_data structure member plat_priv remains uninitialized till
the function invoke of cnss_usb_probe. This leads to the access of
uninitialized pointer plat_priv if CLD gets loaded prior to
firmware download completion. Hence initialize the plat_priv
in cnss_usb_data structure during cnss_usb_init.

Change-Id: Ic471eacf22b112aaffe61458e22c7a9102470467
Signed-off-by: Jayachandran Sreekumaran <jsreekum@codeaurora.org>