cnss2: Fix invalid assignment due to type mismatch

The firmware size returned by request_firmware api is size_t which is
unsigned long in some platforms. Typeacast the size to u32 to match with
supported interface definition.

Change-Id: I61a484e74bf7932508295aac2fe7cfd7298d7953
Signed-off-by: Mahesh Kumar Kalikot Veetil <mkalikot@codeaurora.org>