iwlwifi: runtime: avoid calling debugfs read functions more than once

Upon first calling read() on a debugfs file, invoke
iwl_dbgfs_##name##_read and store the response buffer on the heap, so
subsequent read() calls don't need to invoke said function again.

This is done because cat etc will call read() repeatedly until EOF is
reached (or read() returns 0), which in the current implementation will
cause said function to be invoked multiple times.

With the current implementation this can also cause buggy behavior in
some weird edge cases where the first invocation returns a string of
length n, and the second of length m>n: The last m-n characters of
the second invocation will be printed to screen.

Signed-off-by: Naftali Goldstein <naftali.goldstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>