tools/mrvl_uart.sh: Fix minicom baudrate

minicom doesn't inherit the baudrate from stty but uses its own
defaults, such as for example 57600, whereas we expect 115200 here.
Explicitly tell minicom which baudrate to use.

Fixes: eee4835d ("tools: Add Marvell recovery image download script")
Cc: Konstantin Porotchkin <kostap@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Igal Liberman <igall@marvell.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Konstantin Porotchkin <kostap@marvell.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>