cxgb4: Fixed incorrect check for memory operation in t4_memory_rw

Fix incorrect check introduced in commit fc5ab020

 ("cxgb4: Replaced the
backdoor mechanism to access the HW memory with PCIe Window method"). We where
checking for write operation and doing a read, changed it accordingly.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>