rtase: Fix spelling mistake: "tx_underun" -> "tx_underrun"

There is a spelling mistake in the struct field tx_underun, rename
it to tx_underrun.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240909134612.63912-1-colin.i.king@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>