ath10k: avoid possible memory access violation

array "ctl_power_table" access index "pream" is initialized with -1 and
is raised as a static analysis tool issue.
[drivers\net\wireless\ath\ath10k\wmi.c:4719] ->
[drivers\net\wireless\ath\ath10k\wmi.c:4730]: (error) Array index -1 is
out of bounds.

Since the "pream" index for accessing ctl_power_table array is initialized
with -1, there is a chance of memory access violation for the cases below.
1) wmi_pdev_tpc_final_table_event change frequency is between 2483 and 5180
2) pream_idx is out of the enumeration ranges of wmi_tpc_pream_2ghz,
wmi_tpc_pream_5ghz

Signed-off-by: K.T.VIJAYAKUMAAR <vijay.bvb@samsung.com>
[kvalo@codeaurora.org: clean up the warning message]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>