brcmfmac: fix curly brace mistake in brcmf_pcie_handle_mb_data()

Running coccicheck on brcm80211 drivers resulted in following report:

$ make coccicheck MODE=report M=drivers/net/wireless/brcm80211

  drivers/net/wireless/brcm80211/brcmfmac/pcie.c:595:2-43:
    code aligned with following code on line 596

It revealed that due to a merge failure a block statement lost its
curly braces where it should not.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>