ath10k: remove unnecessary comparison of unsigned integer with < 0

There is no need to compare *ps_state_enable* with < 0 because
such variable is of type u8 (8 bits, unsigned), making it
impossible to hold a negative value.

Fix this by removing such comparison.

Addresses-Coverity-ID: 1473921 ("Unsigned compared against 0")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>