net: eth-phy: Handle gpio_request_by_name() return value

The gpio_request_by_name() returns zero in case of success, however the
conditional return value check in gpio_request_by_name() checks only for
(ret != -ENOENT) and if the condition is true, returns ret outright.

This leads to a situation where successful gpio_request_by_name() return
leads to immediate successful eth_phy_of_to_plat() return as well, and
to skipped parsing of "reset-assert-us" and "reset-deassert-us", so the
PHY driver operates with valid reset GPIO, but with assert/deassert times
set to default, which is 0, instead of the values from DT. This breaks
PHY reset.

Fix this by checking if return value is non-zero and then for this one
single allowed non-zero return value, -ENOENT.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>