obexd/opp: Fix OPP GET request path

obexd/opp: Fix OPP GET request path

The default path is /etc/bluetooth/vcard.vcf which obexd
is not able to access.

obexd[11654]: obexd/src/obex.c:cmd_get() session 0x64edff0
obexd[11654]: GET(0x3), (null)(0xffffffff)
obexd[11654]: obexd/src/obex.c:parse_type() TYPE: text/x-vcard
obexd[11654]: open(/etc/bluetooth/vcard.vcf): Operation not permitted (1)
obexd[11654]: GET(0x3), NOT_FOUND(0x44)

After making this path as obex root folder,
it is able to succeed.

obexd[12246]: obexd/src/obex.c:cmd_get() session 0x64ed830
obexd[12246]: GET(0x3), (null)(0xffffffff)
obexd[12246]: obexd/src/obex.c:parse_type() TYPE: text/x-vcard
obexd[12246]: obexd/src/obex.c:driver_get_headers() name=(null) type=text/x-vcard object=0x8
obexd[12246]: GET(0x3), CONTINUE(0x10)
obexd[12246]: obexd/src/obex.c:send_data() name=(null) type=text/x-vcard file=0x8 size=3061
obexd[12246]: obexd/src/obex.c:driver_read() 73 read
obexd[12246]: obexd/src/obex.c:send_data() name=(null) type=text/x-vcard file=0x8 size=3066
obexd[12246]: obexd/src/obex.c:driver_read() 0 read
obexd[12246]: obexd/src/obex.c:transfer_complete()