efi: stub: Change _debug_uart_putc function to inline

efi: stub: Change _debug_uart_putc function to inline

Update definition of _debug_uart_putc to static inline.
This will allow to avoid compilation warnings about unused code
after introduction of patch changing debug uart functions to
dummies if CONFIG_DEBUG_UART is not set.
This also matches the instructions in include/debug_uart.h and
provides consistency with implementations for other platforms.

Signed-off-by: Lukasz Czechowski <lukasz.czechowski@thaumatec.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>