Update pico-sdk submodule commit point

[caternuson]

The current pico-sdk sudmodule commit point is prior to a fixed added to help with startup timing for the crystal. See here for more information:
raspberrypi/pico-sdk#457

Once updated, this hack should be able to be removed from the CI worflow:

u2if/.github/workflows/build_uf2.yml

Lines 16 to 21 in 945423f

	#-- HACK-----------------------------------------------------------
	# needed for older pico-sdk, see here for more info:
	# https://github.com/raspberrypi/pico-sdk/pull/457
	- name: Patch pico-sdk for crystal startup
	run: sed -i 's/xosc_hw->startup = startup_delay;/xosc_hw->startup = startup_delay * 64;/' firmware/pico-sdk/src/rp2_common/hardware_xosc/xosc.c
	#-- HACK-----------------------------------------------------------

[caternuson]

This may require more work than just updating the pico-sdk submodule. The pico-sdk in turn has tinyusb as a submodule. There have been some breaking changes to tinyusb that will require additional code changes in the u2if source code.