mac80211: Fix clang warning about constant operand in logical operation

When clang detects a non-boolean constant in a logical operation it
generates a 'constant-logical-operand' warning. In
ieee80211_try_rate_control_ops_get() the result of strlen(<const str>)
is used in a logical operation, clang resolves the expression to an
(integer) constant at compile time when clang's builtin strlen function
is used.

Change the condition to check for strlen() > 0 to make the constant
operand boolean and thus avoid the warning.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>