brcmsmac: do a read after the write of the objmem on broken PCIe controllers

As described in the documentation of bcma_wflush16 in drivers/net
/wireless/brcm80211/brcmsmac/types.h some PCIe controllers of Broadcom
SoCs are broken. The PCIe controller on these SoCs are mostly used to
connect some additional wifi device to the SoC and some of these wifi
devices are supported by brcmsmac.
For my BCM43224 connected to the broken PCIe controller of the BCM4718 I
need an extra read after write in brcms_b_write_objmem() to prevent a
Data bus error. This fixes the problem reading tsf_random later.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>