test_firmware: Test partial read support

Add additional hooks to test_firmware to pass in support
for partial file read using request_firmware_into_buf():

	buf_size: size of buffer to request firmware into
	partial: indicates that a partial file request is being made
	file_offset: to indicate offset into file to request

Also update firmware selftests to use the new partial read test API.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201002173828.2099543-17-keescook@chromium.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>