staging: wilc1000: remove function pointer spi_rx of wilc_spi_t

This patch removes function pointer spi_rx of wilc_spi_t and just call
linux_spi_read instead.

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