drivers: Fix uninitialized variable warning.

In GCC 14.2.0, the keyvariable in usb_dc_ep_write is detected to be used
before initialization, even though this usage should be safe.

Fix by initializing the key variable to 0, and making the 2 if
statements explicitly checking the same value.

Signed-off-by: Jeremy Bettis <jbettis@google.com>