rsi: remove set but not used variables 'header_size' and 'tx_params'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/rsi/rsi_91x_hal.c: In function 'rsi_send_data_pkt':
drivers/net/wireless/rsi/rsi_91x_hal.c:288:5: warning:
 variable 'header_size' set but not used [-Wunused-but-set-variable]

'tx_params' only used for 'header_size' dereferenced,so also
can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>