[FIX] sale: fix ecommerce quantity width

Issue

    - Install eCommerce, use Graphene
    - Shop > Any product

    The add to cart button is shifted.

Cause

    Quantity field take full-width & max-width is too large.

Solution

    Remove width: 100% and reduce the max-width.
    Screenshots on OPW

OPW-2166060

closes odoo/odoo#43459

X-original-commit: 48ce80f0
Signed-off-by: Jason Van Malder <jvm-odoo@users.noreply.github.com>