rsi: remove redundant duplicate assignment of buffer_size

Variable buffer_size is re-assigned the same value, this duplicated
assignment is redundant and can be removed.

Cleans up clang warning:
drivers/net/wireless/rsi/rsi_91x_usb.c:140:4: warning: Value stored
to 'buffer_size' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>