ow(4): increase regular mode recovery time, t_rec, to 15 us

Previously we used the minimal value of 1 us and it was really tight.
Application Note 3829 has a table describing recommended t_rec values
for various bus voltages, temperature conditions and numbers of slave
devices. The new value decreases the maximum possible data rate from
16.3 Kbit/s to 13.3 Kbit/s, but it allows for up to four slaves on a
3.3V bus (under room temperature).

References:

• Maxim Integrated Application Note 3829 Determining the Recovery Time for Multiple-Slave 1-Wire(R) Networks

• Maxim Integrated Application Note 937 Book of iButton Standards

Discussed with: imp (D22108)
MFC after: 3 weeks