net: phy: fix use of uninit variable when setting PLCA config

Coverity reported the following:

*** CID 1530573:    (UNINIT)
drivers/net/phy/phy-c45.c:1036 in genphy_c45_plca_set_cfg()
1030     				return ret;
1031
1032     			val = ret;
1033     		}
1034
1035     		if (plca_cfg->node_cnt >= 0)
vvv     CID 1530573:    (UNINIT)
vvv     Using uninitialized value "val".
1036     			val = (val & ~MDIO_OATC14_PLCA_NCNT) |
1037     			      (plca_cfg->node_cnt << 8);
1038
1039     		if (plca_cfg->node_id >= 0)
1040     			val = (val & ~MDIO_OATC14_PLCA_ID) |
1041     			      (plca_cfg->node_id);
drivers/net/phy/phy-c45.c:1076 in genphy_c45_plca_set_cfg()
1070     				return ret;
1071
1072     			val = ret;
1073     		}
1074
1075     		if (plca_cfg->burst_cnt >= 0)
vvv     CID 1530573:    (UNINIT)
vvv     Using uninitialized value "val".
1076     			val = (val & ~MDIO_OATC14_PLCA_MAXBC) |
1077     			      (plca_cfg->burst_cnt << 8);
1078
1079     		if (plca_cfg->burst_tmr >= 0)
1080     			val = (val & ~MDIO_OATC14_PLCA_BTMR) |
1081     			      (plca_cfg->burst_tmr);

This is not actually creating a real problem because the path leading to
'val' being used uninitialized will eventually override the full content
of that variable before actually using it for writing the register.
However, the fix is simple and comes at basically no cost.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Fixes: 49332341

 ("drivers/net/phy: add helpers to get/set PLCA configuration")
Signed-off-by: Piergiorgio Beruto <piergiorgio.beruto@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/f22f1864165a8dbac8b7a2277f341bc8e7a7b70d.1674056765.git.piergiorgio.beruto@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>