sdio: add default c_ispeed/c_ospeed values to sdio_uart driver

Note that the default baudrate is 4800 instead of 9600 as a convenience
because that's what GPS devices want which is still the main use for
this driver.

Signed-off-by: Nicolas Pitre <npitre@mvista.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>