staging: wilc1000: rename spi function names

There are several similar function names, such as wilc_spi_write and
_wilc_spi_write. It is likely to be confused after merging linux_wlan_spi.c and
wilc_spi.c, so rename following functions properly.
Rename wilc_spi_write to wilc_spi_tx, wilc_spi_read to wilc_spi_rx,
wilc_spi_write_read to wilc_spi_tx_rx, _wilc_spi_write to wilc_spi_write,
_wilc_spi_read to wilc_spi_read.

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