tty: n_gsm: add missing description to structs in gsmmux.h

tty: n_gsm: add missing description to structs in gsmmux.h

Currently, all available structure fields in gsmmux.h except those
for gsm_config are commented. Furthermore, no kernel doc comments are used.

Fix this by adding appropriate comments to the not commented fields of
gsm_config. Convert the comments of the other structs to kernel doc format.

Note that 'mru' and 'mtu' refer to the size without basic/advanced option
mode header and byte stuffing as defined in the standard in chapter 5.7.2.

Link: https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230817093231.2317-2-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>