Bluetooth: Fix CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY build warnings

This commit fixes compilation warnings that are present when compiling
with CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY as can be seen in this
compiler log:

"""
In file included from /zephyr-sdk-0.16.1/arm-zephyr-eabi/picolibc/
include/string.h:215,
                 from /zephyr/subsys/bluetooth/host/smp.c:15:
In function '__memcpy_ichk',
    inlined from 'sc_send_public_key' at /zephyr/subsys/bluetooth/host/
smp.c:3006:2:
/zephyr-sdk-0.16.1/arm-zephyr-eabi/picolibc/include/ssp/string.h:83:1:
warning: argument 2 null where non-null expected [-Wnonnull]
   83 | __ssp_bos_icheck3_restrict(memcpy, void *, const void *)
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~
/zephyr-sdk-0.16.1/arm-zephyr-eabi/picolibc/include/ssp/string.h:83:1:
note: in a call to built-in function '__builtin_memcpy'
/zephyr/subsys/bluetooth/host/smp.c: In function 'smp_public_key':
/zephyr/subsys/bluetooth/host/smp.c:4214:21: warning: argument 2
null where non-null expected [-Wnonnull]
 4214 | memcmp(smp->pkey, sc_public_key, BT_PUB_KEY_COORD_LEN) == 0) {
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/zephyr-sdk-0.16.1/arm-zephyr-eabi/picolibc/include/string.h:62:10: note:
in a call to function 'memcmp' declared 'nonnull'
   62 | int memcmp (const void *, const void *, size_t);
      |     ^~~~~~
"""

The warning is caused by the potential use of NULL "sc_public_key"
global pointer that is not assigned a value in "smp_init()" if
CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY is enabled. This commit
conditionally changes the behavior of function "smp_public_key()"
if CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY is activated to simply return
and not use the "sc_public_key" variable. Other functions that are not
called anymore by "smp_public_key()" are also conditionally
deactivated when CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY is enabled

Signed-off-by: Sebastian Panceac <sebastian.panceac@ext.grandcentrix.net>