DuetPkg: exit pci function loops early if device is not multi-function

When looping through all PCI functions, code should not look for functions
1-7 if function 0 is not present or if function 0 indicates the device is
not multifunction.  Prior to this fix the code would use stale data in a
buffer to determine if a device is multifunction even if function 0 is not
present.  This fixes a code bug and provides very small performance
improvements.

PCI 2.3 Specification states: They [multifunction devices] are also
required to always implement function 0 in the device. Implementing other
functions is optional and may be assigned in any order.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Joseph Shifflett <joseph.shifflett@hpe.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18575 6f19259b-4bc3-4df7-8a09-765794883524

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