parisc: Fix out of array access in match_pci_device()

As found by the ubsan checker, the value of the 'index' variable can be
out of range for the bc[] array:

UBSAN: Undefined behaviour in arch/parisc/kernel/drivers.c:655:21
index 6 is out of range for type 'char [6]'
Backtrace:
 [<104fa850>] __ubsan_handle_out_of_bounds+0x68/0x80
 [<1019d83c>] check_parent+0xc0/0x170
 [<1019d91c>] descend_children+0x30/0x6c
 [<1059e164>] device_for_each_child+0x60/0x98
 [<1019cd54>] parse_tree_node+0x40/0x54
 [<1019d86c>] check_parent+0xf0/0x170
 [<1019d91c>] descend_children+0x30/0x6c
 [<1059e164>] device_for_each_child+0x60/0x98
 [<1019d938>] descend_children+0x4c/0x6c
 [<1059e164>] device_for_each_child+0x60/0x98
 [<1019cd54>] parse_tree_node+0x40/0x54
 [<1019cffc>] hwpath_to_device+0xa4/0xc4

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org