drivers: spi: xspi: Modify HW xfer workaround

commit adcf48d421b86eac0cf0dc95c2baa38371aa5ac9 from
git@git.assembla.com:cavium/WindRiver.linux.git

Write data into rx_buffer, with write_len offset.
Xfer will behave as standard xfer() after that change.

Signed-off-by: Witold Sadowski <wsadowski@marvell.com>
Change-Id: I637215d743aea53975a3c532be72288edfd9d25c
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/123255


Reviewed-by: Chandrakala Chavva <cchavva@marvell.com>
Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>