wifi: rtw89: mac: use regular int as return type of DLE buffer request

The function to request DLE (data link engine) buffer uses 'u16' as return
value that mixes error code, so change it to 'int' as regular error code.
Also, treat invalid register value (0xfff) as an error.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230414082228.30766-1-pkshih@realtek.com