hci: llc_shdlc: style: Simplify bool comparison

Fix the following coccicheck warning:
./net/nfc/hci/llc_shdlc.c:239:5-21: WARNING: Comparison to bool

Signed-off-by: YANG LI <abaci-bugfix@linux.alibaba.com>
Reported-by: Abaci <Robot&lt;abaci@linux.alibaba.com>
Link: https://lore.kernel.org/r/1610357063-57705-1-git-send-email-abaci-bugfix@linux.alibaba.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>