tools/mrvl_uart.sh: Fix minicom baudrate

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: eee4835d22 ("tools: Add Marvell recovery image download script")
Cc: Konstantin Porotchkin <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Igal Liberman <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>
Reviewed-by: Konstantin Porotchkin <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>
Signed-off-by: Stefan Roese <[email protected]>