fat: fix utf8 iocharset warning message

The fat_msg function already formats the given message and appends
a newline to it - we don't need to do this in the passed message
string as well, or will end up with a blank line printed in the
kernel log ring buffer.

Also change the loglevel from error to warning.

Signed-off-by: Mihai Moldovan <ionic@ionic.de>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>