rtw88: Fix an error code in rtw_debugfs_set_rsvd_page()

The sscanf() function returns the number of matches (0 or 1 in this
case).  It doesn't return error codes.  We should return -EINVAL if the
string is invalid

Fixes: c376c1fc ("rtw88: add h2c command in debugfs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/YE8nmatMDBDDWkjq@mwanda