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make sure channel request reference is cleared if send fails. without
this change this could happen when att_handle_rsp was called:

1. reqs before call:
head: 0x2000f8e8, tail: 0x2000f8c0, elements:
- addr 0x2000f8e8, function pointer NULL
- addr 0x2000f8c0, function pointer 0x35c1d

2. att_handle_rsp called, calling bt_att_req_free with address
0x2000f8e8

3. reqs after call:
head: 0x2000f8e8, tail:	0x2000f8c0, elements:
- addr 0x2000f8e8, function pointer NULL
- addr 0x2000f8d4, function pointer NULL
- addr 0x2000f8ac, function pointer NULL
- addr 0x2000f898, function pointer NULL
- addr 0x2000f884, function pointer NULL
- addr 0x2000f870, function pointer 0xd92b7e7c
- addr 0x2000f85c, function pointer 0x462a03a9
- addr 0x2000f848, function pointer 0xf77b2f4b
- addr 0x2000f834, function pointer 0x33714775
- addr 0x2000f820, function pointer 0x31ba37f8
- addr 0x2000f80c, function pointer 0x5fda8494
- addr 0x2000f7f8, function pointer 0xbcff174e
- addr 0x2000f7e4, function pointer 0x341393f
- addr 0x2000f7d0, function pointer 0xbcfee8b8
- addr 0x2000f7bc, function pointer 0x1e73d9e5

which obviously is broken.

closes #39506.
Signed-off-by: Jacob Siverskog <jacob@teenage.engineering>