serial: sh-sci: Uninitialized variables in sysfs files

The kstrtol() function returns -ERANGE as well as -EINVAL so these tests
are not enough.  It's not a super serious bug, but my static checker
correctly complains that the "r" variable might be used uninitialized.

Fixes: 5d23188a ("serial: sh-sci: make RX FIFO parameters tunable via sysfs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>