serial: amba-pl011: Respect DMA controller's copy_align requirement

serial: amba-pl011: Respect DMA controller's copy_align requirement

Some DMA controllers require transfer lengths to be aligned to a
specific boundary. For example, the Tegra GPC DMA requires 4-byte
(word) aligned transfers and will reject unaligned lengths.

Align the TX DMA buffer length down to the DMA controller's copy_align
boundary before submitting the transfer. Any remaining unaligned bytes
will be transmitted via PIO on subsequent calls, which is the existing
fallback behavior when DMA is not used.

Signed-off-by: Kartik Rajput <kkartik@nvidia.com>
Link: https://patch.msgid.link/20260225065915.341522-5-kkartik@nvidia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>