ipw2x00: Fix comment for CLOCK_BOOTTIME constant

Correct name of constant is CLOCK_BOOTTIME and not CLOCK_BOOTIME.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200508195139.20078-1-pali@kernel.org