[rp2040] Make writes to SIE_CTRL aware of concurrent access

This commit makes it so that when setting the START_TRANS bit in the
SIE_CTRL register, along with some other bits, we first set all the
other bits, then wait some cycles, and then set the START_TRANS bit.

Doing so protects against a situation where the USB controller is
reading the register at the same time and gets an incorrect value.

This mirrors the procedure already applied to buffer control
registers.