media: dvb-frontends: drx39xyj: Refactor firmware upload

Do not cache the file, instead load it on demand.

This makes smatch a happy parser:
drivers/media/dvb-frontends/drx39xyj/drxj.c:11908 drx_ctrl_u_code() warn: 'fw' from request_firmware() not released on lines: 11877,11886,11896.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>