rsi: USB tx headroom cleanup

USB headroom is added while submitting the data to URB as per
firmware's requirement. This logic is moved to rsi_usb_card_write() so
that caller need not worry about it.

Signed-off-by: Karun Eagalapati <karun256@gmail.com>
Signed-off-by: Amitkumar Karwar <amit.karwar@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>