Even though it is not expected, and would trigger a WARN_ON, killing a
transfer in a uninitialized slot this sequence is warned by clang
analyzer, twice:

drivers/usb/isp1760/isp1760-hcd.c:1976:18: warning: The result of the left shift is undefined because the right operand is negative [clang-analyzer-core.UndefinedBinaryOperatorResult]
                skip_map |= (1 << qh->slot);
drivers/usb/isp1760/isp1760-hcd.c:1983:18: warning: The result of the left shift is undefined because the right operand is negative [clang-analyzer-core.UndefinedBinaryOperatorResult]
                skip_map |= (1 << qh->slot);

Only set skip map if slot is active.
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Link: https://lore.kernel.org/r/20210819180929.1327349-5-rui.silva@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>