cmd: avb read_rb: print rb_idx in hexadecimal

cmd: avb read_rb: print rb_idx in hexadecimal

Prior to this patch was do_avb_write_rb() reading supplied rb_idx as a
hexadecimal number while do_avb_read_rb() printed the read out rb_idx as
decimal number. For consistency change do_avb_read_rb() to print rb_idx
as a hexadecimal number too.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>