mt76x0: Remove VLA usage

Even with "const" variables, the compiler will generate warnings about
VLA usage. In the quest to remove all VLAs from the kernel[1], this uses
a #define instead of a const to do the array sizing.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Fixes: e87b5039

 ("mt76x0: eeprom files")
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>