drivers: led: is31fl3194.c fix uninitialized scalar variable

in the switch case in the preceeding for loop if default path is taken
all the time, then the ret variable will stay uninitialized, the original
contributor of this driver has provided a comment that this path shall
never be reached, however, it is better to return an error code instead
of continuing with an incorrect configuration, hence this commit replaces
continue with a proper return errno.

Signed-off-by: Jilay Pandya <jilay.pandya@outlook.com>