p54: clamp properly instead of just truncating

The call to clamp_t() first truncates the variable signed 8 bit and as a
result, the actual clamp is a no-op.

Fixes: 0d78156e ('p54: improve site survey')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>