[efi] Fix Coverity warning about unintended sign extension

[efi] Fix Coverity warning about unintended sign extension

The result of multiplying a uint16_t by another uint16_t will be a
signed int.  Comparing this against a size_t will perform an unwanted
sign extension.

Fix by explicitly casting e_phnum to an unsigned int, thereby matching
the data type used for the loop index variable (and avoiding the
unwanted sign extension).

This mirrors wimboot commit 15f6162 ("[efi] Fix Coverity warning about
unintended sign extension").

Signed-off-by: Michael Brown <mcb30@ipxe.org>