wifi: rtw89: fw: propagate error code from rtw89_h2c_tx()

The error code should be propagated to callers during downloading firmware
header and body. Remove unnecessary assignment of -1.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250217064308.43559-4-pkshih@realtek.com