sdio_uart: fix sign of paramter status in sdio_uart_receive_chars()

This also fixes a sparse warning about different signedness.
Only compile tested, because i do not have the hardware.

Signed-off-by: Andre Haupt <andre@bitwigglers.org>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>