Fix for "unsafe use of type bool" warning when compiling with MSVC.

[amubiera]

Using negate on a bool causes a warning when compiling with MSVC 19.X:

littlefs\lfs.c(2561): error C2220: the following warning is treated as an error
littlefs\lfs.c(2561): warning C4804: '-': unsafe use of type 'bool' in operation

[geky-bot]

Tests passed ✓, Code: 17112 B (+0.0%), Stack: 1448 B (+0.0%), Structs: 812 B (+0.0%)

	Code	Stack	Structs		Coverage
Default	17112 B (+0.0%)	1448 B (+0.0%)	812 B (+0.0%)	Lines	2429/2591 lines (-0.0%)
Readonly	6222 B (+0.0%)	448 B (+0.0%)	812 B (+0.0%)	Branches	1280/1610 branches (+0.0%)
Threadsafe	17964 B (+0.0%)	1448 B (+0.0%)	820 B (+0.0%)		Benchmarks
Multiversion	17184 B (+0.0%)	1448 B (+0.0%)	816 B (+0.0%)	Readed	29369693876 B (+0.0%)
Migrate	18776 B (+0.0%)	1752 B (+0.0%)	816 B (+0.0%)	Proged	1482874766 B (+0.0%)
Error-asserts	17884 B (+0.0%)	1440 B (+0.0%)	812 B (+0.0%)	Erased	1568888832 B (+0.0%)