ramips: lzma-loader: use proper register names

ramips: lzma-loader: use proper register names

Before this was reworked, in the file for mt7621 subtarget
(target/linux/ramips/image/lzma-loader/src/board-mt7621.c)
the "Transmitter shift register empty" bit TEMT was used instead of
the "Transmitter holding register empty" bit THRE,
but after the rework, this value was labeled as the THRE bit instead.

Functionally there is no difference, but this is confusing to read,
as it suggests that the subtargets have different bits for the same
register in UART when in reality they are exactly the same.

One can use either bit, or both, at user's descretion
in order to determine whether the UART TX buffer is ready.
The generic kernel early-printk uses both,
(arch/mips/kernel/early_printk_8250.c)
while the ralink-specific early-printk uses only THRE,
(arch/mips/ralink/early_printk.c).

Define both bits and rewrite macros for readability,
keep the same values, as changing which to use should be tested first.

Ref: c31319b66 ("ramips: lzma-loader: Refactor loader")
Signed-off-by: Michael Pratt <mcpratt@pm.me>