wifi: rtw89: Correct immediate cfg_len calculation for scan_offload_be

wifi: rtw89: Correct immediate cfg_len calculation for scan_offload_be

Ensures the correct calculation of `cfg_len` prior to the allocation of
the skb in the `rtw89_fw_h2c_scan_offload_be` function, particularly when
the `SCAN_OFFLOAD_BE_V0` firmware feature is enabled. It addresses an
issue where an incorrect skb size might be allocated due to a delayed
setting of `cfg_len`, potentially leading to memory inefficiencies.

By moving the conditional check and assignment of `cfg_len` before skb
allocation, the patch guarantees that `len`, which depends on `cfg_len`,
is accurately computed, ensuring proper skb size and preventing any
unnecessary memory reservation for firmware operations not supporting
beyond the `w8` member of the command data structure.

This correction helps to optimize memory usage and maintain consistent
behavior across different firmware versions.

Fixes: 6ca6b918f280 ("wifi: rtw89: 8922a: Add new fields for scan offload H2C command")
Signed-off-by: Liang Jie <liangjie@lixiang.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250112105144.615474-1-buaajxlj@163.com