MIPS: generic/yamon-dt: fix uninitialized variable error

In the case where fw_getenv returns an error when fetching values
for ememsizea and memsize then variable phys_memsize is not assigned
a variable and will be uninitialized on a zero check of phys_memsize.
Fix this by initializing phys_memsize to zero.

Cleans up cppcheck error:
arch/mips/generic/yamon-dt.c:100:7: error: Uninitialized variable: phys_memsize [uninitvar]

Fixes: f41d2430 ("MIPS: generic/yamon-dt: Support > 256MB of RAM")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>