oem-ibm: drop redundant c_str() in CertHandler I/O calls

Remove redundant c_str() calls when passing file paths to
transferFileData() and readFile() in CertHandler.

The stlcstrParam warning reports unnecessary conversions of
std::string objects to C-style strings when the called functions
already accept std::string-compatible parameters. Removing the
explicit c_str() calls improves readability and follows modern
C++ practices.

Modified two call sites in
oem/ibm/libpldmresponder/file_io_type_cert.cpp:

- transferFileData(filePath.c_str(), ...)
  -> transferFileData(filePath, ...)
- readFile(filePath.c_str(), ...)
  -> readFile(filePath, ...)

No functional change intended.

Change-Id: I04236f19b79fe45d8a5d8110dfdea06d876b33d0
Signed-off-by: Meghana Vangapandu <meghanav@ami.com>

oem/ibm/libpldmresponder/file_io_type_cert.cpp

diff --git a/oem/ibm/libpldmresponder/file_io_type_cert.cpp b/oem/ibm/libpldmresponder/file_io_type_cert.cpp
index fef17c4..c975276 100644
--- a/oem/ibm/libpldmresponder/file_io_type_cert.cpp
+++ b/oem/ibm/libpldmresponder/file_io_type_cert.cpp
@@ -62,7 +62,7 @@
     {
         return PLDM_ERROR_INVALID_DATA;
     }
-    auto rc = transferFileData(filePath.c_str(), true, offset, length, address);
+    auto rc = transferFileData(filePath, true, offset, length, address);
     fs::remove(filePath);
     if (rc)
     {
@@ -83,7 +83,7 @@
     {
         return PLDM_ERROR_INVALID_DATA;
     }
-    auto rc = readFile(filePath.c_str(), offset, length, response);
+    auto rc = readFile(filePath, offset, length, response);
     fs::remove(filePath);
     if (rc)
     {