phy: stm32: work around constant-value overflow assertion

FIELD_PREP() checks that a constant fits into the available bitfield,
but if one of the two lookup tables in stm32_impedance_tune() does
not find a matching entry, the index is out of range, which gcc
correctly complains about:

In file included from <command-line>:
In function 'stm32_impedance_tune',
    inlined from 'stm32_combophy_pll_init' at drivers/phy/st/phy-stm32-combophy.c:247:9:
include/linux/compiler_types.h:517:38: error: call to '__compiletime_assert_447' declared with attribute error: FIELD_PREP: value too large for the field
  517 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                      ^
include/linux/bitfield.h:68:3: note: in expansion of macro 'BUILD_BUG_ON_MSG'
   68 |   BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?  \
  115 |   __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
      |   ^~~~~~~~~~~~~~~~
drivers/phy/st/phy-stm32-combophy.c:162:8: note: in expansion of macro 'FIELD_PREP'
  162 |        FIELD_PREP(STM32MP25_PCIEPRG_IMPCTRL_VSWING, vswing_of));
      |        ^~~~~~~~~~

Rework this so the field value gets set inside of the loop and otherwise
set to zero.

Fixes: 47e1bb6b ("phy: stm32: Add support for STM32MP25 COMBOPHY.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20241111103712.3520611-1-arnd@kernel.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>