ath11k: Correctly check errors for calls to debugfs_create_dir()

debugfs_create_dir() returns an ERR_PTR in case of error, but never a
null pointer. There are a number of places where error-checking code can
accordingly be simplified.

Addresses-Coverity: CID 1497150: Memory - illegal accesses (USE_AFTER_FREE)
Addresses-Coverity: CID 1497158: Memory - illegal accesses (USE_AFTER_FREE)
Addresses-Coverity: CID 1497160: Memory - illegal accesses (USE_AFTER_FREE)
Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200927132451.585473-1-alex.dewar90@gmail.com