spi: rzv2h-rspi: Fix SPDR read access width for 16-bit RX

spi: rzv2h-rspi: Fix SPDR read access width for 16-bit RX

The RZ/V2H hardware manual (section 7.5.2.2.1) specifies that read access
size for the SPI Data Register (SPDR) are fixed at 32 bits. The
RZV2H_RSPI_RX macro for the 16-bit data path used readw(), violating
this requirement.

Switch to readl() for the 16-bit RX path to conform to the hardware
specification.

Fixes: 8b61c8919dff ("spi: Add driver for the RZ/V2H(P) RSPI IP")
Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Felix Gu <ustc.gu@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Link: https://patch.msgid.link/20260610-rzv2h-rspi-v2-1-40c80b4a2c90@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>