media: dvb-frontend/mxl5xx: Refactor struct MBIN_FILE_T and MBIN_SEGMENT_T

Replace the older style one-element array with a flexible array member.
There does not seem to be any allocation for these structs in the code, so
no more code changes are required.

The following cocci warnings are fixed:
drivers/media/dvb-frontends/mxl5xx_defs.h:182:4-8: WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays)
drivers/media/dvb-frontends/mxl5xx_defs.h:171:4-8: WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

)

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