udev: fix permission for Ambiq Micro EVK's

udev: fix permission for Ambiq Micro EVK's

Commit 68e204f1e91a ("udev: Add rules for Ambiq Micro EVK's.") was
initially proposed as http://openocd.zylin.com/3429/ then replaced
by http://openocd.zylin.com/3980/
The initial proposal was for file '99-openocd.rules', in which
MODE="664" was the norm.
After merge of http://openocd.zylin.com/2804/ the new udev rules
in '60-openocd.rules' switched to MODE="660", but the evolution of
the above patch missed this change.

Switch udev rules of Ambiq Micro EVK's to MODE="660" and uniform
them to the rest of the file.

Change-Id: I4b4eea535184ee8569da3264bff4f1fafb5bce4d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 68e204f1e91a ("udev: Add rules for Ambiq Micro EVK's.")
Reviewed-on: http://openocd.zylin.com/6004
Tested-by: jenkins