wlcore: fix 64K page support

In the stable linux-3.16 branch, I ran into a warning in the
wlcore driver:

drivers/net/wireless/ti/wlcore/spi.c: In function 'wl12xx_spi_raw_write':
drivers/net/wireless/ti/wlcore/spi.c:315:1: error: the frame size of 12848 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]

Newer kernels no longer show the warning, but the bug is still there,
as the allocation is based on the CPU page size rather than the
actual capabilities of the hardware.

This replaces the PAGE_SIZE macro with the SZ_4K macro, i.e. 4096 bytes
per buffer.

Cc: stable@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>