tools/mrvl_uart.sh: Tidy script output

Fix a typo in help output (awailable -> available).
Tidy the grammar - not the board connects to a port, we do.

While at it, be consistent in upper-casing the comments.

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>