FAT: remove ifdefs to make the code more readable

ifdefs in the code are making it harder to read.
The use of simple if(vfat_enabled) makes no more code and is cleaner.
(the code is discarded by the compiler instead of the preprocessor.)
NB: if -O0 is used, the code won't be discarded

and bonus, now the code compiles even if CONFIG_SUPPORT_VFAT is not
defined.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>