boards/common/nrf52: fix timer config
boards/common/nrf52: fix timer config

The `channels` member should not be set to the number of hardware
channels *n*, but to *n* - 1 instead. The last channel is implicitly
used in `timer_read()`. Hence out of *n* hardware channels, only *n* - 1
are available to the application.

This fixes a bug introduced by 4d02e1524732b040f5109adbeed416b38ceb8c19
which incorrectly set the channel number to *n* rather than to
*n* - 1.

RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT