usb: gadget: u_ether: Fix host MAC address case

The CDC-ECM specification [1] requires to send the host MAC address as
an uppercase hexadecimal string in chapter "5.4 Ethernet Networking
Functional Descriptor":
    The Unicode character is chosen from the set of values 30h through
    39h and 41h through 46h (0-9 and A-F).

However, snprintf(.., "%pm", ..) generates a lowercase MAC address
string. While most host drivers are tolerant to this, UsbNcm.sys on
Windows 10 is not. Instead it uses a different MAC address with all
bytes set to zero including and after the first byte containing a
lowercase letter. On Windows 11 Microsoft fixed it, but apparently they
did not backport the fix.

This change fixes the issue by upper-casing the MAC to comply with the
specification.

[1]: https://www.usb.org/document-library/class-definitions-communication-devices-12, file ECM120.pdf

Fixes: bcd4a1c4

 ("usb: gadget: u_ether: construct with default values and add setters/getters")
Cc: stable@vger.kernel.org
Signed-off-by: Konrad Gräfe <k.graefe@gateware.de>
Link: https://lore.kernel.org/r/20230505143640.443014-1-k.graefe@gateware.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>