Bluetooth: Increase CONFIG_BT_HCI_TX_STACK_SIZE for BT_SPI

Older value of 256 was introduced in 1.7 times. Testing on
BOARD=96b_carbon with 2 weeks old codebase however showed that TX
thread stack however can grow to 324 bytes. Finally, with the
latest master, following stacks are reported on BLE disconnect
(with CONFIG_INIT_STACKS enabled):

rx stack (real size 1024): unused 452 usage 572 / 1024 (55 %)
tx stack (real size 384):  unused 16  usage 368 / 384 (95 %)

Two outcomes:

1. TX stack needs increase.
2. Over time, the stack usage grows, plus variations in SPI
drivers should also be taken into account.

So, increase the stack size to 416 bytes, to leave some headroom
beyond the immediate values seen on 96b_carbon.

Jira: ZEP-2510

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>