mt76: mt7615: fix compiler warning on frame size

The following error is see from the compiler:

  mt7615/debugfs.c: In function ‘mt7615_ext_mac_addr_read’:
  mt7615/debugfs.c:465:1: warning: the frame size of 1072 bytes is
    larger than 1024 bytes [-Wframe-larger-than=]

The issue is due to allocating a buffer as string storage.

Fix by converting to a dynamical allocation of the buffer.

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>