libmodbus: update to 3.1.11

libmodbus: update to 3.1.11

fixes float endianness issues in 3.1.8.

I use a small, custom application to convert data from my
electricity meter into Modbus format for my inverter and wallbox.
I developed this program against an earlier version of libmodbus
on x86, and it ran flawlessly on my ath79 router (big endian).
After migrating to a Mediatek router (little endian), the program
only outputted huge, meaningless values. However, it ran perfectly
fine on my x86 system (little endian), which is running Fedora
with libmodbus v3.1.11.

I then found several bug reports and the changelogs for libmodbus 3.1.11
that described and resolved my problem. So update openwrt to 3.1.11.

Signed-off-by: Stefan Hellermann <stefan@the2masters.de>