staging: mt7621-pci: fix PCIE_FTS_NUM_LO macro

Add missing parenthesis to PCIE_FTS_NUM_LO macro to do the
same it was being done in original code.

Fixes: a4b2eb91

 ("staging: mt7621-pci: rewrite RC FTS configuration")
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>