ASoC: mchp-spdifrx: Fix uninitialized use of mr in mchp_spdifrx_hw_params()

Clang warns:

../sound/soc/atmel/mchp-spdifrx.c:455:3: error: variable 'mr' is uninitialized when used here [-Werror,-Wuninitialized]
mr |= SPDIFRX_MR_ENDIAN_BIG;
^~
../sound/soc/atmel/mchp-spdifrx.c:432:8: note: initialize the variable 'mr' to silence this warning
u32 mr;
^
= 0
1 error generated.

Zero initialize mr so that these bitwise OR and assignment operation
works unconditionally.

Fixes: fa09fa60385a ("ASoC: mchp-spdifrx: fix controls which rely on rsr register")
Link: https://github.com/ClangBuiltLinux/linux/issues/1797
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230202-mchp-spdifrx-fix-uninit-mr-v1-1-629a045d7a2f@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>