wifi: rtw88: usb: Simplify rtw_usb_write_data

The skb created in this function always has the same headroom,
the chip's TX descriptor size. (pkt_info->offset is set by
rtw_usb_write_data_rsvd_page() to chip->tx_pkt_desc_sz.) Use
chip->tx_pkt_desc_sz directly.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/2479507e-3946-492f-857e-83e54969aad2@gmail.com