w1: fix for loop exit condition in w1_seq_show()

The W1_42_FINISHED_BYTE is 0xFF so the cast means the condition is
never true.

Fixes: d9411e57 ('w1: Add support for DS28EA00 sequence to w1-therm')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>