wifi: wil6210: debugfs: fix info leak in wil_write_file_wmi()

The simple_write_to_buffer() function will succeed if even a single
byte is initialized.  However, we need to initialize the whole buffer
to prevent information leaks.  Just use memdup_user().

Fixes: ff974e40

 ("wil6210: debugfs interface to send raw WMI command")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/Ysg14NdKAZF/hcNG@kili