drivers: w1: Make 1-Wire Skip ROM cmd. optional

For systems with a true dynamic 1-Wire nature, the 1-Wire devices
cannot be defined in the DeviceTree (the number of slaves in
the system is not known during the build time).

In other words, there are no pre-defined 1-Wire devices on the bus in
DeviceTree, and thus the slave_count variable is always zero.

That means the skip_rom functionality will always be called even if
there are multiple devices connected to the bus.

This commit allows the user to override this original behavior and avoid
the skip_rom call by ignoring the slave count variable.

Also, this work preserves full backward compatibility.

Signed-off-by: Pavel Hübner <pavel.hubner@hardwario.com>