Return type is float, so operate on float types everywhere.
Fixes this warning while building for PX4:
../../libraries/AP_Math/AP_Math.cpp: In instantiation of 'float safe_asin(T) [with T = double]':
../../libraries/AP_Math/AP_Math.cpp:56:48: required from here
../../libraries/AP_Math/AP_Math.cpp:44:11: warning: implicit conversion from 'float' to 'double' to match other operand of binary expression [-Wdouble-promotion]
if (v >= 1.0f) {
^
../../libraries/AP_Math/AP_Math.cpp:47:11: warning: implicit conversion from 'float' to 'double' to match other operand of binary expression [-Wdouble-promotion]
if (v <= -1.0f) {
^
2605c7265b