include: env: ti: add default for do_main_cpsw0_qsgmii_phyinit

By default this variable is unset and this causes the test condition to
fail on devices that don't have this defined. Set a default value for
this.

   => boot
   ## Error: "main_cpsw0_qsgmii_phyinit" not defined
   [...]

Fixes: 0d72b0f2 ("include: env: ti: ti_common: Run main_cpsw0_qsgmii_phyinit conditionally")

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>