nrf5x: request HFCLK in USB_EVT_READY to fix post-SoftDevice deadlock

USB_EVT_DETECTED runs hfclk_enable() which, when SoftDevice is not yet
enabled, starts HFXO via direct CLOCK register access. After
sd_softdevice_enable() takes over CLOCK, HFXO is physically off again
and SD's HFCLK reference count is 0.

If USB_EVT_READY is fired post-SD (e.g. on nRF52 via Bluefruit's
usb_softdevice_post_enable() when the pre-SD nrfx_power READY callback
didn't get to run before nrfx_power was uninited), the wait loop
'while (!hfclk_running()) {}' calls sd_clock_hfclk_is_running() which
returns false forever -> deadlock that blocks both USB enumeration and
any further app code on the calling thread.

Call hfclk_enable() right before the wait so HFCLK is requested in
whichever context (SD or direct) is current. hfclk_enable() is
idempotent.

Reproduces reliably with bleuart on Feather nRF52840 Express flashed
via JLink: chip wedges in sd_clock_hfclk_is_running SVC, no USB
enumeration, no BLE advertising. With the fix, USB enumerates and BLE
advertises as expected.

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