net: mac802154: Convert the symbol duration into nanoseconds

Tdsym is often given in the spec as pretty small numbers in microseconds
and hence was reflected in the code as symbol_duration and was stored as
a u8. Actually, for UWB PHYs, the symbol duration is given in
nanoseconds and are as precise as picoseconds. In order to handle better
these PHYs, change the type of symbol_duration to u32 and store this
value in nanoseconds.

All the users of this variable are updated in a mechanical way.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20220201180629.93410-3-miquel.raynal@bootlin.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>