ath11k: Handle errors if peer creation fails

ath11k_peer_create() is called without its return value being checked,
meaning errors will be unhandled. Add missing check and, as the mutex is
unconditionally unlocked on leaving this function, simplify the exit
path.

Addresses-Coverity-ID: 1497531 ("Code maintainability issues")
Fixes: 701e48a4 ("ath11k: add packet log support for QCA6390")
Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201004100218.311653-1-alex.dewar90@gmail.com