samd21: make uart_init() static

[mkj]

Avoids a linker conflict if programs are using that same function name.

Otherwise building a samd21 fork of apollo we get

/usr/lib/gcc/arm-none-eabi/9.2.1/../../../arm-none-eabi/bin/ld: _build/qtpy/obj/../../firmware/src/boards/qtpy/uart.o (symbol from plugin): in function `uart_configure_pinmux':
(.text+0x0): multiple definition of `uart_init'; _build/qtpy/obj/hw/bsp/samd21/family.o (symbol from plugin):(.text+0x0): first defined here
/home/matt/3rd/fpga/apollo/lib/tinyusb/../../firmware/src/uart.h:20:6: error: type of 'uart_init' does not match original declaration [-Werror=lto-type-mismatch]
   20 | void uart_init(bool configure_pinmux, unsigned long baudrate);

[hathach]

Look good, thank you