i2ctransfer: Drop redundant variable arg_idx

arg_idx has the same meaning as getopt's optind global variable, so
drop the former and use the latter. Once getopt() is done parsing the
options, there should be no problem using it for ourselves.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>