Remove extraneous zero initializers to clean up Qurt compiler warnings

[katzfey]

Extraneous zero initializers for global / static variables were removed to get rid of the following Qurt compiler warnings:

../../libraries/AP_RCProtocol/spm_srxl.cpp:78:36: warning: suggest braces around initialization of subobject [-Wmissing-braces]
SrxlTelemetryData srxlTelemData = {0};
^
{}
../../libraries/AP_RCProtocol/spm_srxl.cpp:83:34: warning: suggest braces around initialization of subobject [-Wmissing-braces]
static SrxlDevice srxlThisDev = {0};
^
{}
../../libraries/AP_RCProtocol/spm_srxl.cpp:91:36: warning: suggest braces around initialization of subobject [-Wmissing-braces]
static SrxlReceiverStats srxlRx = {0};
^
{}
../../libraries/AP_RCProtocol/spm_srxl.cpp:91:36: warning: suggest braces around initialization of subobject [-Wmissing-braces]
static SrxlReceiverStats srxlRx = {0};
^
{}

Since this data will end up in .bss and be zeroed out it does not need explicit zero initializers.

[andyp1per]

This is an external library, modifying it makes it hard to integrate newer versions - that's why these have not been fixed previously

[katzfey]

Shoot! I have 3 PR's with changes to the spm_srxl files. Unfortunately, it's a C file that was copied over and given the cpp extension. Then the C++ compiler gives warnings because it has C specific stuff in it.

[katzfey]

@andyp1per Take a look at PR28643: #28643

If that is a more reasonable way to deal with these compiler warnings then I can close this PR.