staging: wilc1000: remove function pointer spi_tx of wilc_spi_t

This patch removes function pointer spi_tx of wilc_spi_t and call
linux_spi_write directly.

Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>